It is certain that in this, as in all questions involving great amounts of property, &c., the statements of partisans should be carefully examined and sifted. This is not an act of great virtue or merit, so long as the questions are subjects of mere contemplation; but when numberless individual interests, great risk of capital, &c., are actually mixed up with them, it requires no common fortitude to take a stand, and maintain it against torrents of popular prejudice, the attacks of deeply-interested persons, and the desertion of estimable and intelligent colleagues. It is this singular position that Mr. Gordon now occupies with regard to the railroad rage. He took it early while others thought with him, and he maintains it now, when he is but " one, and alone"! Though he may call, unheard by the thousands who are busy, scheming, and sharing, and transferring,-by the hundreds of thousands who are watching the hourly issue of new railroad propositions, and the daily fluctuations of railroad-shares, we think there is quite enough in Mr. Gordon's statements and arguments, to demand dispassionate examination by the statesman, the political economist, the great carriers of merchandise and passengers, capitalists disposed to speculate, and, in a more remote degree, the public generally.

Mr. Gordon believes that he has demonstrated the truth of all of the following propositions:-"That the first assertions of railway-engineers, as to the limited velocity of canal-conveyance, has been upset ;-that, according to the immutable laws of motion, the edge-railway can merely reduce surface resistance;—that all the reduction of surface-resistance which can be effected by an edge-railway is not worth making, when the line is anywhere but on a dead level;-that surface-resistance can be reduced by other means; that these other means would be more economical, permit of more traffic, and, consequently, furnish more toll-returns to the proprietors; that a system dangerous to the interests of the public, from its withdrawing repair from existing roads, from its being that of a monopoly, and unavoidably attended with loss of life, could be obviated;—that the velocity of railway-travelling may be attained by other means,—means approved of by several of the most eminent engineers ;that agriculturists might have conveyance much more for their interests than railways, from their exclusive nature, ever can be, and—that the forty millions of pounds proposed to be spent on railways, would prevent the supply of necessary funds for more approved systems of intercourse."

These propositions are of immense importance; and Mr. Gordon has evidently taken very considerable pains to acquire information, both on the road and in the closet, concerning them. The facts he has accumulated he lays before his readers, accompanied by reference to his authorities, and his own conclusions. Up to the present hour he admits of no change in his opinions; and almost the last sentence of his supplement is the repetition of his former affirmation, "that a short time will see the general edge railway-system deprecated, as commercially, agriculturally, and politically hurtful." If this prove to be true, what prodigious and expensive mischief is ignorance or knavery spreading at the present moment, over countries that are considered the foremost in science and intelligence!

QUESTIONS FOR SOLUTION RELATING TO METEOROLOGY, HYDROGRAPHY, AND THE ART OF NAVIGATION *.

BY M. ARAGO.

I HAVE Somewhere read, that an individual was once lamenting, in presence of D'Alembert, that the Encyclopædia had acquired such a vast extent. You would have had much more reason for complaint, replied the philosopher, if we had drawn up a negative Encyclopædia (meaning thereby an Encyclopædia containing a mere indication of things, with which we are unacquainted); for in that case a hundred folio volumes would not have been sufficient.

This reply, I must admit, has hitherto appeared to me to have more point than justice. It is true that the progress of human knowledge shows us daily how far our predecessors were ignorant, and how far we in our turn will appear so to those coming after us; but the greater number of important discoveries have taken place spontaneously, without having been foreseen or suspected by any one. Thus, to cite only two or three examples, D'Alembert's negative Encyclopædia could not have contained the most remote allusion to that important and prolific branch of modern physics, now known under the name of Galvanism, or, as it is more properly called, Voltaic Electricity. The multiplicity of phenomena, likewise, which are produced by the polarization of light, when viewed in relation to its reflection, its ordinary refraction as well as that depending on the action of crystallized plates, would not even be indicated; and the same thing may be said of the theory of luminous interferences, in which the singularity of the results is not less remarkable than their infinite variety.

It must be admitted, however, that apart from those important and rare discoveries which are made from time to time all of a sudden, or at least without any visible preparation, and give a new aspect to certain departments of science, there exist important and well-defined questions, which may be confidently recommended to the notice of observers. Having been recently called by the Academy, to draw up instructions regarding physical phenomena, with a view of being transmitted to the Commander of La Bonite, I soon perceived that the author of a negative Encyclopædia, even when confining himself to what is distinct and definite, would have to indicate an infinitely greater number of blanks than I was at first inclined to believe. It likewise appeared to me that published notices in relation to these were calculated to be of great utility, and that numerous well-informed persons having their time at their disposal, would receive from them an impulse which would change them from passive contemplators into active partisans of science. The readers of the present work are now therefore acquainted with the reasons which have led me to deviate from the ordinary practice, and substitute in the room of some complete theory in astronomy, physics, or mechanics, an article in which almost everything remains to be solved, since it relates either to what we know imperfectly, or to what we are entirely ignorant of. It will remain for them to decide whether questions so drawn up will lead to the advantages I ascribe to them, or whether the trial should be confined to this first attempt. It is right,

*These Questions are part of the instructions mentioned at p. 68 of our present volume, and we are indebted for them to JAMESON'S Philosophical Journal, No. XL., 1836.

however, to inform them that the various questions successively proposed were originally, at least the greater part of them, designed for the officers of a ship (La Bonite), commissioned to convey consular agents to Chili, Peru, and the Philippines; I may add, that it was intended that the circumnavigation of this vessel should commence by the way of Cape Horn, and terminate by that of the Cape of Good Hope.

METEOROLOGICAL PHENOMENA.-In meteorology it is requisite to submit to making observations, which, at the time, are attended with no important result. It is necessary to take care to provide for our successors terms of comparison which we ourselves want, and prepare for them the means of resolving a multitude of important questions, on which it is not competent for us to enter, because the ancients possessed neither barometer nor thermometer. These considerations will suffice to explain our reason for requesting, that, during the whole voyage of La Bonite, note should be taken, both by day and night, and from hour to hour, of the temperature of the air, of the temperature of the surface of the sea, and of the atmospheric pressure. They will likewise authorize us to hope that these observations will continue to be made with the same zeal, of which an example has been given by the officers of L'Uranie, La Coquille, L'Astrolabe, La Chevrette, and Le Loiret. At the same time, if unforeseen circumstances require the omission of part of this labour, it would be desirable that the sacrifice should first be made of what is least essential. The details upon which we are about to enter, seem to us calculated, in such cases, to guide the selection to be made by the commander of the expedition.

OBSERVATIONS DESIGNED TO CHARACTERIZE THE PRESENT STATE OF THE GLOBE IN REGARD TO TEMPERATURE. Has the earth arrived at a permanent state with respect to temperature? The solution of this important question seems to require only the direct comparison of the mean temperatures of the same place, taken at two distant periods. But when we take into account the effects produced by local circumstances, when we consider to what an extent the neighbourhood of a lake, of a forest, of a naked or wooded mountain, of a sandy plain, or one formed of meadows, may modify the temperature, every one will perceive that such thermometrical data alone will not be sufficient; that it is necessary, besides, to ascertain that between the periods in question the country, and even the districts adjoining it, have undergone no important change in their physical aspect and in the nature of their cultivation. It is thus seen that the question becomes singularly complicated, and although numerals are adduced, with sufficient precision to admit of a definite estimate, they become mingled with vague suspicions, which continually throw a scrupulous mind into a state of suspense.

Is there, then, no means of solving the difficulty? These means exist, and are by no means of a complicated nature, for we have only to observe the temperature in the open sea at a great distance from continents. If, for this purpose, we make choice of the equinoctial regions, it is not necessary that the observations should be continued for a series of years; the maxima temperatures observed in crossing the line on two or three occasions will be quite sufficient. In the Atlantic, the extremes of these temperatures, as hitherto determined by numerous navigators, are 80°.6 and 84°.2 of Fahr. Taking into account errors in graduation, every one will perceive, that, with a good instrument, the uncertainty of a single observation of the maximum of temperature in the equatorial parts of the Atlantic Ocean, cannot much surpass a degree, VOL. I. 2 A 5

and that the constancy of the mean of four distinct determinations may be relied on to a small fraction of a degree. Here, then, is a result easy to be obtained, directly connected with the calorific influences on which the temperature of the earth depends, and as much separated as possible from the effects of local circumstances. It ought to form a meteorological gift, which every age should be anxious to bequeath to that which succeeds it. The officers of La Bonite will certainly not neglect this part of their instructions. The excellent instruments with which they are furnished, warrants us to expect all that accuracy and precision which the present state of science demands.

OF THE CALORIFIC ACTION OF THE SOLAR RAYS VIEWED IN THEIR RELATION TO THE SITUATION OF PLACES ON THE GLOBE.-Animated discussions have taken place among meteorologists regarding the calorific effects which the solar rays may produce by means of absorption in different countries. Some adduce the observations that have been made towards the arctic circle, from which this singular consequence seems to result, that the sun has a more powerful heat in high than in low latitudes. Others refuse to admit this result, on the pretence that it is not proved. The observations made at the equator do not appear to them sufficiently numerous to be taken as one of the terms of comparison; and it is thought, besides, that these observations were made under unfavourable circumstances. This investigation might therefore be recommended to the officers of La Bonite. To execute it successfully they would have need of two thermometers, the reservoirs of which, on the one hand, absorb the solar rays unequally, and, on the other, are not too sensible to the cooling influences of currents of air. This double condition may easily be obtained, if, after having procured two thermometers in every respect alike, the bulb of one of them be covered to a certain thickness with white wool, and that of the other with an equal quantity of black wool. These two instruments, exposed to the sun, side by side, will never indicate the same degree; that with the black covering will mount highest. The question, therefore, will consist in determining if the difference of the two indications is less at the equator than at Cape Horn, or at any other higher latitude *.

It will be easily understood that comparative observations of this nature ought to be made at equal altitudes of the sun, and during the most serene weather. Slight differences of altitude, however, will not always impair the accuracy of the observations, if care be taken, under different latitudes, to determine according to what progression the difference of the two instruments increases from sun-rise till mid-day, and diminishes from the latter period till sun-set. Days on which the wind is very high ought to be altogether excluded, whatever be the state of the atmosphere in other respects.

Another observation, somewhat analogous to that of the two thermometers differently covered, will consist in determining the maximum temperature which the sun imparts to a dry soil in equinoctial countries. At Paris, in August 1826, during a serene state of the sky, we found that a thermometer lying horizontally, and having its bulb covered with one millimetre of very fine vegetable mould, stood 129°.2 Fahr. The same instrument, covered to double that depth with river-sand, indicated only 114°.8 Fahr.

*There are other means still more exact for resolving the problem to which the calorific action of the solar rays has given rise; but these depend on instruments which were not to be found in the hands of our artists at the time of the departure of La Bonite, and therefore are not alluded to in the instructions of the Academy. We will return to the consideration of them on another opportunity.

EXPERIMENTS TO BE MADE ON THE RADIATION OF THE SKY.-The experiments which we are about to propose ought to give, all other things being equal, the degree of the atmosphere's transparency. This transparency may be appreciated in a manner in some sort inverse and not less interesting, by observations on nocturnal radiation, which are likewise recommended to the commander of La Bonite.

It has been known for half a century, that a thermometer placed under a clear sky, on the grass of a meadow, indicates 11o, 124°, or even 14° Fahr. less than a thermometer, in every respect similar, suspended in the air, at a few feet from the ground. But it is only a few years since an explanation of this phenomenon was given; for it was only in 1817 that Wells established the fact by means of important experiments, and in a thousand different ways, that this inequality of temperature is caused by the feeble radiating power of a clear sky.

A screen placed between certain solid bodies and the sky prevents them from cooling, because the screen intercepts their radiating communications with the colder regions of the atmosphere. The clouds act in the same manner; they take the place of the screen. But if we distinguish every vapour which intercepts the solar rays coming from above, or the calorific rays ascending from the earth towards the sky, by the name of a cloud, it cannot be said that the atmosphere is ever entirely free from them. The only difference is their greater or less density.

These differences, however slight they may be, may be indicated by the degree of cold to which solid bodies are reduced in the night; and this accompanying peculiarity is worthy of observation, that the transparency measured in this manner, is the mean transparency of the entire firmament, and not that alone of the circumscribed region which may be occupied by a single

star.

In order to make these experiments under the most favourable conditions, it is obvious that we must choose bodies which cool most by radiation. According to the researches of Wells, swan-down is the substance that ought to be selected. A thermometer, having its bulb surrounded with this down, should be placed on a table of painted wood supported by slender feet, in a situation where nothing intercepts the view to the horizon. A second thermometer, with the bulb naked, should be suspended in the air at some height above the ground. With regard to the latter, a screen will secure it from all radiation towards the sky. In England, Wells obtained a difference of 15° Fahrenheit between the indications of two thermometers placed in the manner described. It would certainly be strange, if less important differences were to result from them in equinoctial countries, which have been so much praised for the purity of their atmosphere. It is doubtless unnecessary for us to demonstrate the utility that would attach to such experiments, if they were repeated on a very high mountain, such as Mowna-Roa, or Mowna-Kaah, in the Sandwich Islands.

EXAMINATION OF AN ANOMALY WHICH ATMOSPHERIC TEMPERATURES, TAKEN AT DIFFERENT ELEVATIONS, PRESENT IN THE NIGHT, WHEN THE SKY IS CALM AND CLEAR.-The temperature of atmospheric strata diminishes in proportion as these strata become more elevated. There is only one exception to this rule, and that is observed in the night during a calm and clear state of the air. In these circumstances, an increasing progression takes place, to a