« AnteriorContinuar »
series Sof exact hyperirhood of its n the sodium ian the wavehutter for use ght star in the
secure better agnet, adjustjat it can be tes round the vagnet the star n for parallax ed at Dunsink le of the field, attached to the is no vibration e moving parts. oil may control lescope, so that, ered.-Prof. T.
of the peculiar lor, Harv., and the specific charhe structure of the a list of the rarer
cruise of the s.s. 1891). One fish, deep water off the The following are Linn.), from 500 to
Raia microcellata iyo and Donegal ; w water-Donegal was again taken ; proved to be abunnoms. The follow
inhabiting littoral o fathoms : Scyllium aris, Raia oxyrhyn
-M. Duchartre in the phosphides of boron, ints out that he ren and phosphorus in a 1 more fully described erefore claims priority note on the subject on ential equations, by M. he adiabatism of a conenty.-The vapour ten
Georges Charpy. The at different temperatures untaining 32 per cent. of 040°, and from 75o on
Each of these right lines ydration of the salt; the i solution, the upper of a hose of M. Etard, but the
states is from 40° to 75° y this observer, a difference itions in his experiments. — nium and potassammonium, ulation of the temperature of fatty acids, by M. G. Hintive malic acid and potassium sol. The heat of solution of 4 litres), - 3:31 Cal. ; heats Cal., by Na = + 24.86 Cal.; salts :
5.78 Cal. ces = + 1'55 Cal.
166 Cal. res = + 1978 Cal. ts indicate that malic acid lies i in the energy of its action.
The elongation, the former, below Pole, later. In this way the for producing such fringes, by providing the cap of the objective right ascension of Polaris plays a small part in its azimuth of with two parallel slits, adjustable in width and distance apart. elongation, which is dependent solely on the declination and If such a combination be focussed on a star, then, instead of the latitude. Assuming the present declinations of the two stars concentric rings before mentioned, there will be a series of mentioned, with probable errors of < =0":2 and 0":3 re. straight equidistant bands whose length is parallel with the spectively, he finds that the right ascension would probably be slits, the central one being brightest,' Fig. 1, c. in error by + 0.002s. and = 0'0043. In fact, the probable The general theory of these fringes may be found in the
dependent upon anything but the transit of the star to Philosophical Magazine for March 1891. The general equation be determined will be much less if the present method is used showing the relation between the visibility of the fringes and the (with an equal instrument), than is stars in the same declination, distance between the slits is: but opposite Polaris in right ascension, were observed by direct comparisons in the meridian.” By applying this method to
kx dx other stars of different right ascensions and gradually increas.
V?= ing declinations," as the R.A. of Polaris or its opposite is approached, numerous co-ordinates thoroughly independent can he obtained, and will “provide zero points for the proposed number of photographic plates 2° square, and consequently help
which reduces to the simpler form 10) settle the places of all stars in that region.”
(x) cos kx dx V=
(2) MEASUREMENT OF JUPITER'S SATELLITES
(x) dx BY INTERFERENCE.
when the object viewed is symmetrical. It has long been known that even in a telescope which is A number of applications of this formula are discussed in the
theoretically perfect, the image of a luminous point is composed of a series of concentric circles with a bright patch of confined to the case in which the object viewed (or rather its
former paper, but for the present purpose attention will be light at the common centre. This system of circles can easily projection) is a circular disk, uniformly illuminated. be observed by examining any bright star with a telescope pro
In this case equation (2) becomes vided with a circular diaphragm which diminishes the effective aperture. The appearance of the image is shown in Fig. 1, a. In the case of an object of finite angular magnitude the image
(3) could be constructed by drawing a system of such rings about every point in the geometrical image. The result for a small disk (corresponding to the appearance of one of the satellites of in which a is the angular diameter of the object, and an is the Jupiter as seen with a 12-inch telescope whose effective aperture smallest angle resolvable by an equivalent aperture ; that is, the
ratio of a light-wave to the distance between the slits. Fig 1
The curve expressing this relation is given in Fig. 2, in which the ordinates are values of the visibility of the fringes, and the abscissæ are the corresponding values of the alao.
has been reduced to six inches) is given in Fig. 1, b; the chief points of difference between this and Fig. 1, a, being the greater size of the bright central disk, and the lesser clearness of the sur. rounding rings. The larger the disk the more nearly will the appearance of the image correspond to that of the object ; and the smaller the object the more nearly does it correspond with Fig. 1, a, and the more difficult will be the measurement of its actual size. Thus, in the case just cited, the actual angular diameter is about one second of arc, and the uncertainty may amount to half this value or even more.
The relative uncertainıy, other things being equal, will be less in proportion to the increaso in the aperture, so i hat with the 36-inch telescope the measurement of the diameters of Jupiter's satellites should be accurale io within ten per cent. under favourable conditions.
It is important to note that in all such measurements the image observed is a diffraction phenomenon-the rings being interference fringes, and the ettings being made on the position of that part of a fringe which is most easily identified. But such measurements must vary with the atmospheric conditions and especially with the observer—for no two observers will agree upon the exact part of the fringe to be measured, and the uncertainties are exaggerated when the fringes are disturbed by atmospheric tremors.
If, now, it be possible to find a relation between the size of the object and the clearness of the interference fringes, an independent method of measuring such minute objects will be furnished ; and it is the purpose of this paper to show that such a method is not only feasible, but in all probability gives results
In to Astronomical Measurements” | an arrargement was described
' Philosophical Magazine, July 1890.
ao whence, putting s for the distance between the centres of the slits, and taking for the wave-length of the brightest part of the spectrum o'0005 mm.,' and dividing by the value of a second in radians we have
In consequence of the kind invitation extended by Prof. Holden, it was decided to make a practical test of the usefulness of the proposed method at Mount Hamilton.
* These will be superposed on another set of fringes due to diffraction from the edges of the slits; but the latter are too faint and broad to cause any confusion.
? The wave-length will, of course, vary somewhat with the object observed, but may be made constant by interposing a red glass.
For the preliminary experiments which are to be described it the disappearance could still be sharply marked. Indeed the was thought desirable to use the 12-inch equatorial. Accordingly, concordance of the observations made under different circuma cap, provided with two adjustable slits, was fitted over the stances on different nights was even closer than was expected. objective, and provided with a rod by means of which the distance With a larger telescope both the brightness of the fringes and between the slits could be altered gradually and at will by the their distance apart will be increased, and it may be confidently observer, while the distance was measured on a millimetre scale predicted that the accuracy will then be even greater. attached to the sliding jaws. This arrangement, which was con- The values given in the second column, Engelmann,” are structed under the supervision of Mr. F. L. O. Wadsworth, probably more reliable than the succeeding ones, but it is well of Clark University, is shown in the accompanying diagram, worth noting that the results obtained by interference agree with Fig. 3.
the others quite as well as these agree with each other.
It should also be noted that the distance between the slits was Figo
about four inches. It may therefore be stated that for such measurements as have just been described, a telescope sufficiently large to admit a separation of four inches—say a six-inchsuitably provided with adjustable slits is fully equal to the largest telescopes now used without them.
It is hoped that within a few months the 36-inch equatorial will be supplied with a similar apparatus and observations begun for the definite measurement of the satellites of Jupiter and Saturn and such of the asteroids as may come within the range of the instrument.
In concluding, I wish to take this opportunity of expressing my appreciation of the courtesy of Director Holden in placing all the facilities of the Observatory at my disposal, and of the hearty co-operation of all the astronomers of the Observatory, especially the valuable assistance of Prof. W. W. Campbell in making the observations.
A. A. MICHELSON. Mount Hamilton.
THE SAMOAN CYCLONE OF MARCH 16, 1889.
historic storms that have been rendered for ever memorable by the episodes of disaster and gallantry that attended them; by the escape of H.M.S. Calliope, which forced her way out of Apia harbour in the teeth of the hurricane, amid the cheers of the brave American sailors, who, themselves face to face with imminent death, forgot for a moment their own dire peril in their admiration of the daring and successful act of seamanship that rescued their more fortunate brothers. Mr. Everard Hayden, of the U.S. Hydrographic Office, has lately issued a preliminary Report on this storm, which, despite the regrettable meagreness of the data at his command, has, nevertheless, a
certain scientific interest, inasmuch as less is known of the With this apparatus the satellites of Jupiter were measured, cyclones of the Pacific than of those of most other tropical with results as given in the following table :
The Apia storin, like the cyclones of the South Indian Ocean, TABLE I.
was evidently formed on the northern limits of the south-east No. of Satellites.
Seeing. trades, and was one of a series that were generated in this region
in March 1889. The first of these, in Mr. Hayden's opinion, August 2 1-29 1:19
appears to have originated on the 5th of the month, some 500 August 3 I'29
miles north-north-east from the Samoan Islands, and to have 1'59 1.68
Poor. August 6 1*30 1'69 1'56
travelled first in a south-westerly direction, recurving in the
latitude of these islands, but at 150 to 200 miles to the west of 1°77
them, after which it took a south-eastward course between Tonga Mean... 1°29 1:19
and Nuië. It seems to have been a storm of great severity, and 1'73 166
its passage was felt at Apia on the 6th and 7th, though not with These are the values of the angular diameters of the satellites any great intensity. It was succeeded by the cyclone that forms of Jupiter as seen from the earth. To reduce these to Jupiter's the principal subject of Mr. Hayden's Report.' This, be thinks, mean distance these values are to be multiplied by 079, which was formed about March 13, some 300 miles to the north-east of gives for the final values
the Samoan Islands, and on the 15th its centre passed either 1.
directly over, or a little to the north of, A pia harbour, moving, II.
therefore, on a south-west course. He considers that it then
sharply recurved, and that, with greatly increased strength, it For the sake of comparison these values are recorded in the passed a second time over Apia on ihe 16th, the day of the great following table, together with those given by Engelmann, naval disaster. The chief facts which led Mr. Blayden to Struve, and Hough, and the last column contains some results this conclusion are those observed at Apia itself, for no positive kindly furnished by Prof. Burnham with the 36-inch on the evidence is forthcoming from the supposed birthplace of the same date (August 7) as the last of the series by A. A. M, :- storm, and only one ship reports the state of the weather any.
where to the north of Samoa. The peculiar feature of the Apia TABLE II.
observations is, that the barometer fell steadily from the 12th to
Bu, the afternoon of the 15th (about 0'7 inch), then rose (about 0-25 I...
inch) during the latter part of that day, and then again fell on the 11. 0.94 o'91 o'91 0.98
16th to a reading slighily lower than that of the previous day. III. 1937 I'54 I'49
1978 1978 On the 15th, squalls of moderate force (wind southerly, sorce 2 IV.
barometer rose, the direction changed from south to north and It was found impossible to see the reappearance of the fringes east. There had been no heavy sea, and it was thought that on increasing the distance, yet the results of Table I. show that the gale was over. At midnight, however, the barometer began
falling again, the wind had increased, and the sea was high. for the Loundean Professor of Astionomy for the Lent and The barometer continued falling, and the gale rapidly developed
Easter Terms. its full strength. From early morning of the 16th, for nearly We regret to hear that Prof. Adanıs's health does not yet twenty-four hours, it blew a hurricane, and the catastrophes allow him to resume his duties. commenced with the loss of the Eber.
Any attempted interpretation of facts so meagre must necessarily be in a great measure speculative. We have given that of Mr. Ilayden, and others have been suggested. One, that of
SOCIETIES AND ACADEMIES. Lieutenant Wiizel, is to the effect that the storm of the 16th was
LONDON distinct from that of the previous day, and originated over Savaii (the island to the west of Upolu, in which is the harbour
Royal Society, November 19.—"The Thermal Emissivity of Apia). Another, by Mr. Dutton, is that the storm of the
of Thin Wires in Air.” By W. E. Ayrton, F.R.S., and H. 15th, after approaching the Samoan Islands on a south-west Kilgour. track, recurved to west and north-west, and during the following
In 1884 it was observed experimentally that whereas the night again recurved sharply, describing a loop north of Savaii,
electric current required to maintain a thick wire of given and then returning towards Upolu, whence it moved southwards
material, under given conditions, at a given temperature was and south eastwards. Our own interpretation is somewhat approximately proportional 10 the diameter of the wire raised different from any of these, and seems to be more in accordance
to the power ihree halves, the current was more nearly prowith the habits of tropical cyclones, the movements of which are
portional to the first power of the diameter if the wire were thin. by no means so erratic as that implied by Mr. Dutton's hypo
When this difference in the behaviour of a thick and thin wire thesis, while it does not involve the extremely and, we think,
was first noticed, it was regarded as being quite unexpected. improbably sharp recurvature suggested by Mr. Hayden, nor
But, as pointed out by one of us in the course of a discussion at the equally improbable generation of a second vortex cnly one
a meeting of the Royal Society, the unexpected characier of the day in the rear of the storm of the 15th, as supposed by result was due to people having assumed that the loss of heat Lieutenant Witzel. None of these explanations seem to take
from radiation and convection per square centimetre of sursace account of the circumstances that attend the formation of tropical per 1° excess temperature was a consiant, and independent of the cyclones, which, as we have elsewhere pointed out, differ in size and shape of the cooling body. many respects from the storms of the temperate zone.
The very valuable investigations that have been made on It is evident from the considerable and steady fall of the emissivity by Mr. Macfarlane, Prof. Tait, Mr. Crookes, Mr. barometer at A pia from March 12 to 15, that the Samoan J; T. Bottomley, and by Mr. Schleiermacher, had for their Islands lay within the area of disturbance in which the storm
object the determination of the variation of the emissivity with was generated, and that the formation of the vortex was simply changes of the surface and with change in the density of the gas the concentration of this disturbance, which probably took place surrounding the cooling body, but it was not part of these nearer to Apia than is supposed by Mr. Hayden, but still at
investigations to determine the change in the emissivity that is such a distance that the first effect of the concentration-viz. a produced by change in the shape and size of the cooling body. slight rise of the barometer in the area immediately around, and
Indeed, so litile has been the aitention devored to the very large especially on the polar side-was felt at the Samoan Islands. If, change that can be brought about in the value of the emissivity then, as seems probable, the vortex was not formed until the by simply changing the dimensions of the cooling body, that in afternoon of the 15th, this, in conjunction with the ordinary Prof. Everett's very valuable book of “Units and Physical Condiurnal rise between 4 and 10 p.m., would account for the slight
stants” the absolute results obiained by Mr. Macfarlane are given rise observed at Apia on the latter part of that day, and only the
as the “results of experiments on the loss of heat from blackened second fall to a minimum on the 16th was due to the actual
and polished copper in air at almospheric pressure," and no passage of the cyclone. From the severity of the storm, as felt
reference is made either to the shape or to the size of the cooling at Apia harbour, it is clear that Upolu must have been traversed
body. by at least a portion of the inner vortex, but it could bardly
[November 19, 1891.-Since this paper was sent in to the have been very close to the centre, seeing that the barometer Royal Society a new edition of this book has appeared, and, in never fell to 29 inches; and therefore the long duration of the consequence of a suggestion made to Prof. Everett, the word hurricane (24 hours) can only be explained by the very slow rate
"balls” has been added after the word "copper" in this new at which the storm was then travelling. This slow rate of pro
edition, as well as the following paragraph :gression strengthens the probability that it had not proceeded
" Influence of Size.- According to Prof. Ayrton, who quotes far from its birthplace, since, as a rule, tropical cyclones move
a table in ‘Box on Heat,' the coefficient of emission increases forward slowly at first, and only gradually acquire greater speed
as the size of the emiting body diminishes, and for a blackened of translation. It also strengihens the inference that it had sphere of radius r centims. may be stated originated not very far to the north or north-east of Upolu.
'0003609. This explanation, as already remarked, can only be regarded
'0004928 + as tentative, but it seems at least worthy of consideration by those who may have fuller data at hand.
H. F. B. The value of r in Macsarlane's experiments was 2."]
The laws which govern the lo.s of heat from very thin cylindrical conductors have not only considerable scientific interest in
showing how the shape of a body affects the convection currents, UNIVERSITY AND EDUCATIONAL
but they are os especial importance to the clectrical engineer in INTELLIGENCE.
connection with glow lamps, hot-wire volimelers, suses, &c. We
theresore thought it desirable to ascertain the way in which the Cambridge.—The Sheep-hanks Astronomical Exibition has | law of cooling for thick wires, which involved the diameter raised been awarded 10 P. H. Conell, Scholar of Trinity College. to the power ihree halves, passed into the law for the cooling of
A memorial signed by 107 members of the Senate is published thin wires, involving only the first power of the diameter. For by the Vice-Chancellor : il expresses the opinion that "the this objeci, the investigation described in the paper was comwhole question of degrees in science should be considered by menced at the beginning of 1888, and the emissivity was the University.” Among the signatures are some of those who measured of nine platinum wires, having the diameters of 1'2, took the Greek as well as some who took the anti-Greek side in 290, 29, 40, 60, 8:1, 9-3, III, and 14 mils, or thousandths the recent controversy.
of an inch. In view of the fall in the aggregate of the Colleges the Suspecting that some of the published results on the currents Council of the Senate propose to obtain powers for deferring the required to suse wires had been much influenced by the cooling next increment of the College contribution to the University action of the blocks to which the ends of the wires were attached, from 1893 to 1895, and the following increment (from £25,000 we started by making a calculation of the length necessary to to £30,000) for seven years further-namely, to 1903.
give to our wires, so that ihe loss of heat by conductior. should Sir George Gabriel Stokes and Prof. Macalister, M.D., not introduce any important error into the determination of the are among the delegates appointed to represent the University emissivity. To do this it was necessary to calculate the disat the Dublin Tercentenary Festival next year.
tribution of temperature along a wire through which a steady Mr. E. W. Holson, of Chri·t's College, is approved as Deputy current was flowing, and from which he al was lost by radiation, convection, and conduction, and it was further necessary to remainder of whose tube had been simply calibrated for improve on the calculation one of us had published on this uniformity of bore. The consequence was that when we subject in the Electrician for 1879, by taking into account the desired to compare one of our thermometers reading, say, from fact that the emissivity, as well as the thermal and electric con- 200° to 300° C., with the Kew standard, their bulbs were very ducting power, of the wire differed at different points in con- far apart when both were immersed in the oil-bath, and with sequence of the difference of temperature.
the tops of the mercury columns just above the surface of the Until we had completed the experiments described in this oil. A short description is given in the paper of the devices paper, we could, of course, only employ in this calculation values' employed to overcome this difficulty, and which enabled an that we had guessed at as being something near the truth for accurate comparison to be made between the thermometers. the emissivity of platinum wire for different diameters and at On examining the curves, accompanying the complete paper, different temperatures. Hence, after the completion of the ex- which show the emissivity for each temperature for each of the periments, we took up the mathematical investigation again, nine wires, we see thatsubstituting for the einissivity such a function of the diameter of (1) For any given temperature the emissivity is the higher the the wire and the temperature of the point as we had experiment- finer the wire. ally found it to be. Section IV. of the paper contains the in- (2) For each wire the emissivity increases with the temperature, vestigation by which we finally arrived at the calculated dis- and the rate of increase is the greater the finer the wire. For tribution of temperature along the wire, and we have to express the finest wire the rate of increase of emissivity with temperature our sincere thanks to Prof. Henrici (whom we consulted as to is very striking. the best method of practically solving the rather complex (3) Hence ihe effect of sursace on the total loss of heat (by differential equation arrived at) for the warm interest that he has radiation and convection) per second, per square centimetre, per taken in the mathematical treatment of the subject, and for the , 1° C. excess temperature, increases as the temperature rises. many suggestions which he has made, and which have enabled On comparing the loss of heat from the wire of 12 mils us to arrive at the mathematical solution given in the paper. diameter when at 300° C. with that from the wire of 6 mils
Each wire to be tested was stretched along the axis of a hori- diameter when at 15° C., both being in an inclosure at 10° C., we zontal water-jacketed cylinder 32'5 cm. long, 7-62 cm. external see that the former loses per square centiinetre of surface per and 5.8 cm. internal diameter, the inner surface of which was second not blackened and kept at a constant temperature by a stream of cold
300-10 water flowing through the jacket. The rate at which heat was
or 58 times
15-10' lost by any one of the wires was measured by the product of the current passing through it into the P.D. (potential difference) as much heat as the latter, as it would if the emissivity were the maintained between its ends, wbile the ratio of the P.D. to the same ; but, instead, current gave the resistance of the wire, and, therefore, its
60 x 58, or 3840 times temperature. Experiments were in this way made with various currents flowing through each of the nine wires.
as much heat ; arising from the fact that the emissivity-that is, As the variation of resistance with temperature is known to the number of calories (gramme C.°) lost per second, per square vary with different specimens of platinum, experiinents were centimetre of surface, per 1° C. excess temperature-of the 1'2 separately made to determine the actual law of variation of mil wire at 300° C. is 60 times as great as that of the 6 mil wire resistance with temperature up to 300° C. for each piece of wire at 15°, the emissivity of the latter wire varying very rapidly that had been employed in the emissivity experiments.
near 15°C. In this later deiermination various thermometers were used, From the curves which accompany the complete paper, each and the subsequent comparison of these thermometers with a curve giving the variation of emissivity with temperature for a Kew standard thermometer involved a vast amount of labour, particular wire, the following table has been drawn up, giving from the fact that it is, or at any rate was not possible three the emissivities of the various wires at eight useful temperatures, years ago, to purchase from the Kew Observatory a standard and it will be observed that, in consequence of our investigation thermometer reading srom, say, 200° to 300° C., with a short, having been made on wires of which the thickest was thinner wide chamber at the base in which the mercury expanded below than the thinnest ever previously used in absolute determinations 200°C. All that could be obtained was a long thermometer of emissivity, the emissivities we have experimentally obtained which had been carefully tested between oo and 100° C., and the are far greater than any previously arrived at.
The wire of 4 mils diameter is omitted from the table, as the experiments showed that its specific resistance was much greater, its temperature coefficient much smaller, and its emissivity much smaller than if it had been of platinum. This piece of wire probably, therefore, contained iridium or silver.
We find that the emissivity of thin platinum wires of different temperature above that of the surrounding envelope is prodiameters at the same temperature can be very fairly expressed portional to the diameter of the wire raised to ihe power by a constant plus a constant into the reciprocal of the diameter three halves, is equivalent to stating that the emissivity is of the wire. For example, we find that
independent of the diameter. Now from the three formulæ
(1), (2), (3), given above fore, we may concludeAt 100° C. e = 0'0010360 + O'OT 20776/-?,
That for a temperature of 100° C. the value of d in the ( = 0'0011113 + 0*01430282-1,
(2) formula » 300 € = 0'0011353 + 0'016084 d'",
e = 0'0010360 + 0'01207764-1 where d is the diameter of the wire in mils, or thousandths of must be something like 220 mils, or 5'6 mm., in order that the an inch.
neglect of the second term may not make an error in e of more The statement, not unfrequently made, that the current than 5 per cent., and something like I'15 inch, or 29-3 mm., if required to maintain a wire of a given material at a given the error is not to exceed i per cent.