justment may be easily avoided, by allowing the mercury to play freely between two horizontal surfaces of wood, of moderate extent, and at the distance of one-seventh of an inch: the height may then be always measured from the upper surface, without sensible error. But if the surfaces were closer than this, the mercury would stand too high in the tube.

The same method which is employed for determining the relation between the heights and densities of elastic fluids, may be extended to all bodies which are in any degree compressible, and of which the elasticity is subjected to laws similar to those which are discoverable in the air and in other gases: and it is not improbable that these laws are generally applicable to all bodies in nature, as far as their texture will allow them to submit to the operation of pressure, without wholly losing their form. Water, for example, has been observed by Canton to be compressed one-twenty-two thousandth of its bulk by a force equal to that of the pressure of the atmosphere; consequently this force may be represented by that of a column of water 750 thousand feet in height; the density of the water at the bottom of a lake, or of the sea, will be increased by the pressure of the superincumbent fluid; and suppos. ing the law of compression to resemble that of the air, it may be inferred that at the depth of 100 miles, its density would be doubled; and that at 200 it would be quadrupled. The same. measures would also be applicable to the elasticity of mercury. But there is reason to suppose that they are in both cases a little too small.

[Young's Nat. Phil.

Temperature of the Atmosphere.

THAT the temperature of the air varies considerably, not only in the different climates and in different seasons, but even in the same place and in the same season, must be obvious to the most careless observer. This perpetual variation cannot be ascribed to the direct heat of the sun; for the rays of that luminary seem to produce no effect whatever upon air, though ever so much concentrated but they warm the surface of the earth, which communicates its heat to the surrounding atmosphere. Hence its happens that the temperature of the air is highest in those places which are

so situated as to be most warmed by the sun's rays, and that it varies in every region with the season of the year. Hence, too, the reason why it diminishes according to the height of the air above the sur. face of the earth. That portion of the earth which lies at the equator is exposed to the most perpendicular rays of the sun. Of course it is hottest, and the heat of the earth diminishes gradu. ally from the equator to the poles. The temperature of the air must follow the same order. The air, then, is hottest over the equator, and its temperature gradually diminishes from the equator to the poles, where it is coldest of all. It is hottest at the equator, and it becomes gradually colder according to its height above that surface. Let us examine the nature of these two diminishing pro gressions of temperature.

1. Though the temperature of the air is highest at the equator, and gradually sinks as it approaches the pole; yet as, in every place, the temperature of the air is constantly varying with the season of the year, we cannot form any precise notion of the pro gression without taking the temperature in every degree of latitude for every day of the year, and forming from each a mean temperature for the whole year; which is done by adding together the whole observations, and dividing by their number, The quotient gives the mean temperature for the year. The diminution from the

pole to the equator takes place in arithmetical progression: or, to speak more properly, the annual temperature of all the latitudes are arithmetical means between the mean annual temperature of the equator and the pole. This was first discovered by Mr. Meyer; and by means of an equation which he founded on it, but rendered considerably plainer and simpler, Mr. Kirwan has calculated the mean annual temperature of every degree of latitude between the equator and the pole. And according to this calculation the mean temperature of the equator is 84°, and that of the pole 31°. To find the mean temperature for every other latitude, we have only to find 88 arithmetical means between 84 and 31. In this manner Mr. Kirwan calculated the following Table:

TABLE of the Mean Annual Temperature of the Standard Situation in every Latitude.

This Table, however, only answers for the temperature of the atmosphere of the ocean. It was calculated for that part of the Atlantic Ocean which lies between the 80th degree of northern and the 45th of southern latitude, and extends westward as far as the Gulf-stream, and to within a few leagues of the coast of America; and for all that part of the Pacific Ocean reaching from lat. 45° north to lat. 40° south, from the 20th to the 275th degree of longitude east of London. This part of the ocean Mr. Kirwan calls the standard: the rest of the ocean is subject to anomalies which will be afterwards mentioned.

Mr. Kirwan has also calculated the mean monthly temperature of the standard ocean. The principles on which he went were these: The mean temperature of April seems to approach very nearly to the mean annual temperature; and as far as heat de

pends on the action of the solar rays, the mean heat of every month is as the mean altitude of the sun, or rather as the sine of the sun's altitude. The mean heat of April, therefore, and the sine of the sun's altitude being given, the mean heat of May is found in this manner: As the sine of the sun's mean altitude in April is to the mean heat of April, so is the sine of the sun's mean altitude in May to the mean heat of May. In the same manner the mean heats of June, July, and August, are found; but the rule would give the temperature of the succeeding months too low, because it does not take in the heat derived from the earth, which possesses a degree of heat nearly equal to the mean annual temperature. The real temperature of these months therefore must be looked upon as an arithmetical mean between the astronomical and terrestrial heats.

Mr. Kirwan, however, after going through a tedious calculation founded upon this principle, found the results to agree so ill with observation, that he drew up an extensive table of the monthly mean temperature of the standard from latitude 80° to lat. 10o, from which it appears that January is the coldest month in every latitude, and that July is the warmest month in all latitudes above 48°. In lower latitudes August is generally warmest. The dif ference between the hottest and coldest months increases in proportion to the distance from the equator. Every habitable latitude enjoys a mean heat of 60° degrees for at least two months; this heat seems necessary for the production of corn. Within ten degrees of the poles, the temperatures differ very little, neither do they differ much within ten degrees of the equator; the temperatures of different years differ very little near the equator, but they differ more and more as the latitudes approach the poles.

2. That the temperature of the atmosphere gradually diminishes, according to its height above the level of the sea, is well known. Thus the late Dr. Hutton of Edinburgh found that a thermometer, kept on the top of Arthur's Seat, usually stood three degrees lower than a thermometer kept at the bottom of it. Hence, then, a height of 800 feet occasioned 3° of diminution of temperature. On the summit of Pinchinca the thermometer stood at 30°, as observed by Bouguer, while at the level of the sea in the same latitude it stood at 84°. Here a height of 15,564 feet occasioned a diminu tion of temperature amounting to 54°. But though there can be

no doubt of the gradual diminution of temperature, according to the height, it is by no means easy to determine the rate of diminu. tion. Euler supposes it to be in a harmonic progression; but this opinion is contradicted by observations. Saussure supposes, that in temperate climates the diminution of temperature amounts to 1° for every 287 feet of elevation. But Mr. Kirwan has shown that no such rule holds, and that the rate of diminution varies with the temperature at the surface of the earth. We are indebted to this philosopher for a very ingenious method of determining the rate of diminution in every particular case, supposing the temperature at the surface of the earth known *.

Since the temperature of the atmosphere is constantly diminish. ing as we ascend above the level of the sea, it is obvious, that at a certain height we arrive at the region of perpetual congelation. This region varies in height according to the latitude of the place; it is highest at the equator, and descends gradually nearer the earth as we approach the poles. It varies also according to the season, being highest in summer and lowest in winter. M. Bouguer found the cold on the top of Pinchinca, one of the Andes, to extend from seven to nine degrees below the freezing point every morning immediately before sun-rise. He concluded, therefore, that the mean height of the term of congelation (the place where it freezes during some part of the day all the year round) between the tropics was 15,577 feet above the level of the sea; but in lat. 28° he placed it in summer at the height of 13,440 feet. Now, if we take the difference between the temperature of the equator and the freezing point, it is evident that it will bear the same proportion to the term of congelation at the equator, that the difference between the mean temperature of any other degree of latitude and the freezing point bears to the term of congelation in that latitude. Thus the mean heat of the equator being 84°, the difference between it and 32 is 52; the mean heat of latitude 28° is 72.3", the dif ference between which and 32 is 40·3: Then 52: 15577 :: 40·3 : 12072. In this manner Mr. Kirwan calculated the following Table:

Irish Trans. viii. 356.