ATMOSPHERE. 



atmosphere on the base of a square foot ; 

 and consequently the 144th part of this, 

 or 15 Ib, is the weight "of 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 13 tons, and when in 

 the heaviest, it is about 14 tons and one- 

 third ; the difference of which is about 

 2464 Ib. 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 lisllessness and inactivity. 

 Hence invalids suffer in their health from 

 very sudden changes in the atmosphere. 

 In 9ur observations on the barometer, we 

 have known the mercury to vary a full 

 inch, o\ % 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 6T 

 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, 01 

 by those who descend to great depths in 

 a diving-bell. See DIVING-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 seert 

 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 



