Book III. 



OF THE ATMOSPHERE. 



349 



i 



2278. Those changes in the atmosjyhere which constitute the most important meteorological 

 j)henomena, may be classed under five distinct heads ; the alterations that occur in the 

 weight of the atmosphere ; those that take place in its temperature ; the changes produced 

 in its quantity by evaporation and rain ; the excessive agitation to vvhich it is frequently 

 subject ; and the phenomena arising from electric and other causes, that at particular 

 times occasion or attend the precipitations and agitations alluded to. All the above 

 phenomena prove to demonstration that constant changes take place, the consequences of 

 new combinations and decompositions rapidly following each other. 



2279. With respect to the changes in the weight of the atmosphere, it is generally known 

 that the instrument called the barometer shows the weight of a body of air immediately 

 above it, extending to the extreme boundary of the atmosphere, and the base of which is 

 equal to that of the mercury contained within it. As the level of the sea is the lowest 

 point of observation, the column of air over a barometer placed at that level is the longest 

 to be obtained. 



2280. The variations of the harometer between the tropics are very trifling, and it does not descend 

 more than half as much in that part of the globe for every two hundred feet of elevation as it does be- 

 yond the tropics. The range of the barometer increases gradually as the latitude advances towards the 

 poles, till in the end it amounts to two or three inches. The following Table will explain this gradual 

 increase : 



2281. The range of the barometer is considerably less 

 in North America than in the corresponding latitudes 

 of Europe, particularly in Virginia, where it never 

 exceeds 11. The range is more considerable at the 

 level of the sea than on mountains ; and in the same 

 degree of latitude it is in the inverse ratio of the 

 height of the place above the level of the sea. Cotte 

 composed a table, which has been published in the 

 Journal de Physique, from which it appears extremely 

 probable, that the barometer has an invariable ten- 

 dency to rise between the morning and the evening, 

 and that this impulse is most considerable from two 

 in the afternoon till nine at night, when the greatest 

 elevation is accomplished ; but the elevation at nine 

 differs from that at two by four twelfths, while that of two varies from the elevation of the morning only 

 by one twelfth, and that in particular climates the greatest elevation is at two o'clock. The observations 

 of Cotte confirm those of Luke Howard ; and from them it is concluded, that the barometer is influenced 

 by some depressing cause at new and full moon, and that some other makes it rise at the quarters. This 

 coincidence is most considerable in fair and calm weather ; the depression in the interval between the 

 quarters and conjunctions amounts to one tenth of an inch, and the rise from the conjunctions to the 

 quarters is to the same amount. The range of this instrument is found to be greater in winter than in 

 summer ; for instance, the mean at York, during the months from October to March inclusive, in the 

 year 1774, was 1*42, and in the six summer months 1'016. 



2282. The more serene and settled the weather, the higher the barometer ranges ; calm weather, with a 

 tendency to rain, depresses it ; high winds have a similar effect on it ; and the greatest elevation occurs 

 with easterly and northerly winds ; but the south produces a dirictly contrary effect. 



2283. The variations in the temperature of the air in any particular place, exclusive of 

 the differences of seasons and climates, are very considerable. These changes cannot be 

 produced by heat derived from the sun, as its rays concentrated have no kind of effect on 

 air ; those, however, heat the surface of our globe, which is communicated to the imme- 

 diate atmosphere ; it is through this fact that the temperature is highest where the place 

 is so situated as to receive with most effect the rays of the sun, and that it varies in each 

 region with the season ; it is also the cause why it decreases in proportion to the .height 

 of the air above the surface of the earth. The most perpendicular rays falling on the 

 globe at the equator, there the heat of it is the greatest, and that heat decreases gradually 

 to the poles, of course the temperature of the air is in exact unison ; from this, it appears, 

 that the air acquires the greatest degree of warmth over the equator, where it becomes 

 insensibly cooler till we arrive at the poles; in the same manner, the air immediately 

 above the equator cools gradually. Though the temperature sinks as it approaches the 

 pole, and is highest at the equator, yet as it varies continually with the seasons, it is im- 

 possible to form an accurate idea of the progression without forming a mean temperature 

 for a year, from that of the temperature of every degree of latitude for every day of the 

 year, which may be accomplished by adding together the whole of the observations and 

 dividing by their number, when the quotient will be the mean temperature for the year. 

 The "diminution," says Dr. Thomson, "from the pole to the equator takes place in 

 arithmetical progression ; or to speak more properly, the annual temperature of all tlie 

 latitudes are arithmetical means between the mean annual temperature of the equator and 

 the pole. And as far as heat depends in the action of solar rays, that of each month is as 

 the mean altitude of the sun, or rather as the sine of the sun's altitude. 



2284. Inconsiderable seas, in temperate and cold climates, are colder in winter and 

 warmer in summer than the main ocean, as they are necessarily under the influence of 

 natural operations from the land. Thus the Gulf of Bothnia is generally frozen in 

 winter, but the water is sometimes heated in the summer to 70, a state, the opposite 

 part of die Atlantic never acquires ; the German Sea is five degrees warmer in summer 

 than the Atlantic, and more than three colder in winter ; the Mediterranean is almost 



