Imágenes de páginas
PDF
EPUB

of winds, &c., in the Port of London, and has improved the calculations by taking into account the moon's motion in an inclined orbit, which had not before been attended to..

Mr. Whewell, of Cambridge, (whose high mathematical ability, and comprehensive turn of mind, had led him to the most general views of the relations of the different branches of Science,) had, at the same time with Mr. Lubbock, been turning his thoughts to the subject of the Tides. He had forcibly remarked that “the theory of the Tides is now in the state which that of the moon and planets presented about a century ago ; and unless considerable exertions be made, it may so continue for many years to come. The tables of the planets have only acquired their present accuracy through the liberal encouragement of learned bodies, and of some of the governments of Europe: nor can tables of the tides adapted to the present state of Science be now constructed, unless a very considerable expense be incurred from the immense labour required."

We have already referred to some of the systems of observations which have been set on foot with such praiseworthy diligence for securing the most accurate information on which to ground these calculations and tables. The zeal of Mr. Whewell has animated a number of other observers; and owing, in a great measure, to the circulation of notices, rules for observing, &c., by that gentleman, and the stimulus and encouragement which has been given to the subject at the meetings of the British Association, we believe that a number of able labourers are now engaged in the work of obtaining data in different places, which will contribute materially to the formation of accurate tables, and enabling us to verify, with some degree of precision, the indications of theory.

In the Philosophical Transactions, 1833, Part I., appeared Mr. Whewell's “Essay towards a first Approximation to a Map of Co-tidal Lines.” We have already stated that a chart of this kind had been given by Mr. Lubbock. Even in the short interval since that time, however, so much had been done in collecting observations, that Mr. Whewell found himself in a condition to give a far more complete and accurate map of the actual position of those lines running in a variety of irregular curves, yet all preserving a certain degree of symmetry and dependence upon each other across the surface of the ocean, and extending up into the lesser seas, which mark the connected series of points at which it is high water at the same instant of time, as at 12, 1, 2, &c., o'clock; a distinct line being laid down for each hour. The forms which these lines assume, then present the track which is followed, as it were, by the continuous crest or ridge of the great tide-wave; and it is impossible to inspect Mr. Whewell's chart without having at once conveyed to the mind a far more complete apprehension of the mode in which this striking effect is propagated throughout the seas, than could possibly be done by any verbal description. Nevertheless, the author, with that caution which distinguishes the true philosophic inquirer, has only ventured to call it a first approximation; as, doubtless, future and more extensive observations will introduce many corrections in the details.

From what we said at first, it will be evident that in the varied courses assumed by the different branches of the Tide, there must be great complexity in the smaller seas, broken and interrupted by promontories and islands; and in none more so than in those seas which surround the British islands. To follow out all these particulars, as far as they can be collected from observations in all parts of the world, and to compare them as well as we can with what theory would indicate, is the object of Mr. Whewell in the profound and elaborate paper to which we refer. Of these details, we shall, of course, not attempt to give any idea. The author has, however, followed them up by other researches in the Philosophical Transactions for 1834, Part I., entitled “On the Empirical Laws of the Tides in the Port of London;" in which he discusses at length, the present state both of theory and observation.

We have alluded to the stimulus of late given to the prosecution of Tide observations. At the end of his paper, in 1833, Mr. Whewell had suggested, that if simultaneous observations could be made on the heights and times of the tide for an entire fortnight or half lunation, at different

points of the coast, the most valuable accession would be made to our • knowledge. The stations of the Coast-guard seemed to offer every facility

for the purpose. Accordingly, a representation being made to that effect to the superintending officers of that department, it was met with the utmost readiness and promptitude. Under the directions of Captain Bowles, the chief commissioner, and Captain Beaufort, the hydrographer of the Admiralty, a plan was immediately organized, in accordance with which, observations at the same time were made at all the stations along the coast of England, Scotland, and Ireland, uninterruptedly, from June 7th to June 22nd (1834), inclusive. These valuable observations were all reported to a common centre; and the various calculations which then had to be made upon them were in a great measure executed by M. Dessiou, of the Admiralty. By their means, Mr. Whewell has been able to lay down, with increasing accuracy, the course of those branches of the great tide-wave coming from the Atlantic, which divides into several minor branches, and occasions so great a complexity in tracing out the real origin of the Tides at different parts of our coasts. These researches, however, throw much new light on the subject, and have enabled the author to pursue the comparison with theory to a far greater extent than he was able to do before. They are detailed in the first part of the Philosophical Transactions for 1835.

But for the still further prosecution of the inquiry, co-operation with foreign states was requisite; and a representation to this effect having been laid before the Lords of the Admiralty by Captain Beaufort, the subject has been taken up by their lordships, and applications to foreign powers have been made for a combined prosecution of such observations in different parts of the world.

At the Dublin meeting of the British Association, Mr. Whewell gave a public address on the subject; in the course of which he dwelt with peculiar emphasis on the ready and liberal spirit of scientific co-operation thus manifested: “In every case,” he observed, “these applications were cordially met; and there was not a maritime state in Europe, not one north of the equator, that was not contributing its assistance to this great work. Sweden, Denmark, Russia, Spain, France, Holland, and the United States, had all joined in it. By the next meeting, he hoped that

[ocr errors]

the inquiries commenced under these favourable auspices would lead to some valuable results.”

We must not omit to add, that in the same address he adverted to an important discovery, closely connected with the theory, which had been announced to the physical section, by Mr. Russell, a zealous and scientific engineer, connected with the canal navigation of Scotland. That gentleman, in the course of a number of valuable trials, on a large scale, with regard to the motion of water in canals, had discovered that the velocity with which a wave is propagated, has a certain determinate mathematical relation to the depth of the water. Such a law must manifestly be of great value in showing what ought to be the influence of the varying depth of the ocean on the motion of the tide-wave.

THE JUVENILE PHILOSOPHER.

No. I. Many of our younger friends are apt to say, “I should very much like to see experiments in Natural Philosophy, and to try them myself; but then it will be necessary to have a variety of curious instruments and expensive apparatus, which are out of my reach :” we shall, therefore, devote an occasional page to showing our young philosopher what a great number of interesting experiments he may perform with no other apparatus than a few of the most common articles, which are everywhere at hand, and the exercise of a very little patience and perseverance in learning to follow the directions which we shall give him.

How to make a Prism.—Take two little bits of broken window-glass, and a lump of wax. After having softened and moulded the wax, stick the two bits of glass upon it, so that they meet together at an angle, as represented in the annexed sketch (fig. 1.) Where w is the wax, g and g' Fig. 1.

Fig. 2.

[graphic][ocr errors][ocr errors]

the glasses stuck to it. , The end view, or section of it (fig. 2), will show the angle a, at which they meet. Into the angle thus made by the glasses, put a drop of water : it will stay there by what is called “capillary attraction. This drop of water is represented by d.

Now to use the instrument which you have made, you should have a small hole in the upper part of the window-shutter, or still better, a narrow horizontal slit, so that you can see the white clouds or sky through it, when you stand at some distance from it in the room. If you cannot manage this, a sheet of pasteboard stuck up in the upper part of the sash with a slit cut in it will do. The slit should be about one-tenth of an

[ocr errors]

inch wide, and an inch or two long, with very even and straight edges. Then holding your prism in your hand, (and for convenience you may stick 昼 ho

a small handle on to the wax,) place it Fig. 3.

close to your eye, and, looking through the drop of water, you will see what is called the “spectrum;" that is, a beautiful train of colours,--at one end red, at the other violet, and in the middle yellowish

green. To give you a better idea of the !

position of the whole, and the direction

in which to look, we subjoin another figure. Here e is the eye of the spectator, p is the prism, h the hole in the shutter, s the spectrum.

Now when you have tried this two or three times, you will get the knack of easily looking in the right direction, and will see the colours very bright and distinct. If you wish to make it more than a mere amusement, and are desirous to learn something from it, write down what colours you see, and in what order. Observe what takes place if you turn the prism about a little; and write that down also. Particularly (if you can) get some idea of the length to which the coloured spectrum extends, compared with its breadth. Thus you may, with a little practice, manage to compare it with some fixed object which may be seen by the side of it, by looking past the prism at the same time. This, perhaps, will be difficult: at any rate, bear in mind the general appearance of the whole ; and, if you can, draw a sketch of it. This will be particularly instructive, if you next take out the drop of water, and put in its place a drop of some other liquid,-oil, for instance. You will now see a spectrum with similar colours; but when you compare your sketch, in this instance, with. the former, you will see a great difference in the length to which the colours are drawn out.

And not only will they differ in this respect, but also the whole spectrum, bodily, will be shifted into a different position ; viz., lower down from the hole or slit than in the former case. If you usc any common oil this will be very evident; but if you procure a little oil of aniseed, or oil of cassia, it will be exhibited in a most striking and peculiar manner. With these substances the spectrum will be stretched out, as it were, into a very great length compared with that formed by the drop of water. You may also try a variety of other fluids. Almost every one, if carefully examined, will offer some peculiarities worthy of notice.

But this method is not confined to fluids. There are even some transparent substances of a more solid nature, which may (with a little dexterity) be used in the way described. For instance, a small piece of gum, or turpentine, may be pressed and squeezed into the angle formed by the two glasses, till it forms a small prism, in the same way as the drop of liquid did before. In this way, the properties of a number of different substances may be examined and compared. We suppose all along that the glasses are kept at the same inclination to each other. If this be altered, the length of the spectrum will be changed owing to this cause alone. You may try the same drop of water, and vary the inclination of the glasses.

RECENT RESEARCHES ON THE FORMATION

OF RAIN.

In a climate like ours, the Science of Meteorology, and especially that department of it which relates to rain, possesses a peculiar interest.

It had been generally understood by philosophers, that the formation of rain was simply due to any sudden condensation of the moisture of the atmosphere at a great height, which so condensed, falls in drops. The air holds always, more or less vapour in solution, in the state of an invisible elastic fluid, but if cooled to a certain degree, this is condensed in the form of visible vapour, that is, fog or cloud; and according to various conditions (chiefly electrical), this is either suspended in that state by the mutual repulsion of its particles, or they are attracted together into drops of larger size, and fall by their gravity in showers of greater or less force, according to the quantity, the elevation, and other circumstances.

The attention of observers has lately been called to this subject, chiefly in consequence of some remarkable anomalies which were noticed. It was commonly observed that more rain falls in mountainous countries and elevated places. This is explained by the circumstance, that the elevations both attract the clouds, and from their greater coldness, promote the deposition. It was therefore a fact apparently quite anomalous, which was noticed in some observations of M. Arago, that at the top of the observatory at Paris, the quantity of rain which fell was considerably less than on the ground. Some observations of the same kind, made by Dr. Heberden, on the roof of Westminster Abbey, compared with a position near the ground, fully confirmed the result. This curious fact, bearing so strongly on the various questions connected with the constitution of the atmosphere, and the formation of its depositions, attracted the notice of several observers in this country, especially Mr. Phillips, the able and scientific secretary of the Yorkshire Philosophical Society, since so worthily appointed Professor of Geology in King's College. This gentleman had turned his attention, among other subjects, to that of meteorology, and found a powerful spirit of cooperation in other individuals, connected with the Yorkshire Society, to assist him in carrying on observations for this express object.

At the first meeting of the British Association, for the advancement of science at York, 1831, the committee, amongst those various other suggestions and recommendations, from which have emanated so many valuable researches and reports, adopted a resolution,“ that Mr. Phillips, and Mr. W. Gray, jun., of York, be requested to undertake a series of experiments, on the comparative quantities of rain falling on the top of the great tower of York Minster, and on the ground near its base. The committee have been induced to propose this specific question, in consequence of the local fitness of the situation, and the facilities offered for its solution by the authorities : but it is to be wished that similar experiments should be made elsewhere, that by an extended comparison of observations, light may be thrown on the anomalies which have been observed at Paris, and in other places.”

This recommendation was ably and zealously seconded, both by the exertions of the scientific individual who undertook the experiments, and

« AnteriorContinuar »