ATMOSPHERE. 



all animal and vegetable life, and even to 

 the constitution of all kinds of matter 

 whatever, without which they would not 

 be what they are : for by it we literally 

 may be said to live, move, and have our 

 being : by insinuating itself into all the 

 pores of bodies, it becomes the great 

 spring of almost all the mutations to which 

 the chemist and philosopher are witnesses 

 in the changes of bodies. Without the 

 atmosphere no animal could exist ; vege- 

 tation would cease, and there would be 

 neither rain nor refreshing dews to mois- 

 ten the face of the ground; and though 

 the sun and stars might be seen as bright 

 specks, yet there would be little enjoy- 

 ment of light, could we ourselves exist 

 without it. Nature, indeed, and the con- 

 stitutions and principles of matter, would 

 be totally changed, if this fluid were want- 

 ing. 



The mechanical force of the atmosphere 

 is of great importance in the affairs of 

 men, who employ it in the motion of 

 their ships, in turning their mills, and in 

 a thousand other ways connected with the 

 arts of life. It was not till the time of 

 Lord Bacon, who taughl his countrymen 

 how to investigate natural phenomena, 

 that the atmosphere began to be investi- 

 gated v.ith am degree of precibiun. Ga- 

 lileo introduced the study by pointing 

 oui its weight. ; a subject that was soon 

 after investigated more completely by 

 Tovricelh and others. Its density and 

 elasticity were ascertained by Mr. Boyle 

 and the academicians at Florence. Mari- 

 otte measured its dilatibility ; Hooke, 

 Newton, Boyle, and Derham, shewed its 

 relation to light, to sound, and to electri- 

 city. Sir I. Newton explained the effect 

 produced upon it by moisture, from which 

 Halley attempted to explain the changes 

 in its weight indicated by the barometer. 

 The atmosphere, we have said, enve- 

 lops the whole surface of the earth, and 

 if they were both at rest, then the figure 

 of the atmosphere would be globular, be- 

 cause all the parts of the surface of a fluid 

 in a state of rest must be equally removed 

 from its centre. But as the earth and the 

 surrounding parts of the atmosphere re- 

 volve uniformly together about their axis, 

 the different parts of both have a centri- 

 fugal force, the tendency of which is more 

 considerable, and that of the centripetal 

 less, as the parts are more remote from 

 the axis, and hence the figure of the at- 

 mosphere must become an oblate sphe- 

 roid, since the parts that correspond to 

 the equator are farther removed from the 

 xis than the parts which correspond to 

 the poles. The figure of the atmosphere 



must also, on another account, represent 

 a flattened spheroid, namely, because the 

 sun strikes more directly the air which 

 encompasses the equator, and is compre- 

 hended between the two tropics, than 

 that which pertains to the polar regions : 

 hence it follows, that the mass of air, or 

 part of the atmosphere adjoining to the 

 poles, being less heated, cannot expand 

 so much nor reach so high. Neverthe- 

 less, as the same force which contributes 

 to elevate the air diminishes its gravity 

 and pressure on the surface of the earth, 

 highter columns of it about^the equatorial 

 parts, other circumstances being the 

 same, may not be heavier than those 

 about the poles. Mr. Kirwan observes, 

 that in the natural state of the atmos- 

 phere, that is, when the barometer would, 

 every where at the level of the sea, stand 

 ut 30 inches, the weight of the atmos- 

 phere at the surface of the sea must be 

 equal all over the globe ; and in order to 

 produce this equality, as the weight pro- 

 ceeds from its density and height, it must 

 be lowest where the density is greatest, 

 and highest where the density is least ; 

 that is, highest at the equator and lowest 

 at the poles, with the intermediate grada- 

 tions. On this and other accounts, in the 

 highest regions of the atmosphere, the 

 denser equatorial air not being supported 

 by the collateral tropical columns, gradu- 

 ally flows over and rolls down to the 

 north and south ; these superior tides 

 have been supposed to consist of hydro- 

 gen gas, inasmuch as it is much lighter 

 than any other, and is generated in great 

 plenty between the tropics ; it is also 

 supposed to furnish the matter of the 

 aurorae borealis and australis. 



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 34$ feet 

 high ; and the cubical foot of water weigh- 

 ing just 1000 ounces, or 62 pounds, 

 34 multiplied by 62 or 2158 Lb. will be 

 the weight of a column of water, or of the 



