With regard to the weight and pressure of the atmosphere, it is evident that the whole mass, in common with all other matter, must be endowed with weight and pressure : and it is found, by undeni able experiments, that the pressure of the atmosphere sustains a column of quick silver in the tube of a barometer of about 30 inches in height ; it accordingly fol lows, that the whole pressure of the at mosphere is equal to the weight of a co lumn of quicksilver of an equal base, and 30 inches in height, or the weight of the atmosphere on every square inch of sur face is equal to 15 pounds. It has more over been found, that the pressure of the atmosphere balances, in the case of pumps, &c. a column of water 34i feet high ; and the cubical foot of water weigh ing just 1000 ounces, or 62i pounds, 343- multiplied by 62. or 2158 lb. will be the weight of a column of water, or of the atmosphere on the base of a square foot ; and consequently the 144th part of this, or 15 lb. is the the atmosphere on a square inch. From these data, Mr. Cotes computed the pressure of the at mosphere on the whole surface of the earth to be equivalent to that of a globe of lead sixty miles in diameter. Dr. Vince and others have given the weight at 77670297973563429 tons. This weight is however variable ; it sometimes being much greater than at others. If the sur face of a man, for instance, be equal to 14 square feet, the pressure upon him, when the atmosphere is in its lightest state, is equal to 13i tons, and when in the heaviest, it is about 14 tons and one third ; the difference of which is about 2464 lb. It is surprising that such weights should be able to be borne without crush ing the human frame : this indeed must be the case, if all the parts of our body were not endowed with some elastic spring, whether of air or other fluid, suf ficient to counterbalance the weight of the atmosphere. Whatever this spring is, it is certain that it is just able to counteract the weight of the atmosphere, and no more ; of course it must alter in its force as the density of the atmos phere varies ; for if any considerable pres sure be superadded to that of the air, as by going into deep water, it is always se verely felt ; and if, on the other hand, the pressure of the atmosphere be taken off from any part of the human body, by means of the apparatus belonging to the air pump, the inconvenience is imme diately perceived.
The difference in the weight of the at mosphere is very considerable, as has been observed, from the natural changes in the state of the air. These changes take place chiefly in countries at a dis tance from the equator. In Great Britain, for instance, the barometer varies from 28.4 to 30.7. On the increase of this na tural weight, the weather is commonly clear and fine, and we feel ourselves alert and active ; but when the weight of the air diminishes, the weather is often bad, and we feel listlessness and inactivity. Hence invalids suffer in their health from very sudden changes in the atmosphere. In our observations on the barometer, we have known the mercury to vary a full inch, or even something more, in the course of a few hours. Such changes, how ever, are by no means frequent. Ascend ing to the tops of mountains, where the pressure of the air is very much diminish ed, the inconvenience is rarely felt, on ac count of the gradual change ; but when a person ascends in a balloon with great rapidity, he feels, we are told by Garne rin and other aeronauts, a difficulty Or breathing, and many unpleasant sensa tions. So also on the condensation of the
air, we feel little or no alteration in our selves, except when the variations are sudden in the state of the atmosphere, or by those who descend to great depths in a diving-bell. See DiviNn-BELL.
It is not easy to assign the true reason for the changes that happen in the gravi ty of the atmosphere in the same place. One cause is, undoubtedly, the heat of the sun ; for where this is uniform, the changes are small and regular. Thus, between the tropics the barometer con stantly sinks about half an inch every day, and rises to its former station in the night. But in the temperate zones, the altitude of the mercury is subject to much more considerable variations, as we have seed' with respect to what is observable in our own country.
As to the alteration of heat and cold, Dr. Darwin infers that there is good reason to conclude, that in all circumstances where air is mechanically expanded, it becomes capable of attracting the fluid matter of heat from other bodies in contact with it. Now, as the vast region of air which sur rounds our globe is perpetually moving along its surface, climbing up the sides of mountains, and descending into the val leys, as it passes along, it must be per petually varying the degree of heat ac cording to the elevation of the country it traverses ; for in rising to the summits of mountains it becomes expanded, having so much of the pressure of the super incumbent atmosphere taken away ; and when thus expanded, it attracts or ab sorbs heat from the mountains in conti guity with it ; and when it descends into the valleys, and is compressed into less compass, it again gives out the heat it has acquired to the bodies it comes in contact with. The same thing must happen in the higher regions of the atmosphere, which are regions of perpetual frost, as has lately been discovered by the aerial navigators. When large districts of air, from the lower parts of the atmosphere, are raised two or three miles high, they become so much expanded by the great diminution of the pressure over them, and thence become so cold, that hail or snow is produced by the precipitation of the vapour : and as- there is, in these high regions of the atmosphere, nothing else for the expanded air to acquire heat from after it has parted with its vapour, the same degree of cold continues till the air, on descending to the earth, acquires its former state of condensation and of warmth. The Andes, almost under the line, rests its base on burning sands : about its middle height is a most pleasant and temperate climate, covering an ex tensive plain, on which is built the city of Quito ; while its forehead is encircled with eternal snow, perhaps coeval with the mountain. Yet, according to the ac count of Don Ulloa, these three discord ant climates seldom encroach much on each other's territories. The hot winds below, if they ascend, become cooled by their expansion ; and hence they cannot affect the snow upon the summit ; and the cold winds, that sweep the summit, become condensed as they descend, and of temperate warmth before they reach the fertile plains of Quito.