ATM I 



Atmo- roidal like itself. " All the atmospheric strata," 

 sphere. sa y S L a place, " should take after a time the rota- 

 " "V ->' t ory motion, common to the body which they sur- 

 round. For the friction of these strata against each 

 other, and against the surface of the body, should ac- 

 celerate the slowest motions, and retard the most 

 rapid, till a perfect equality is established among 

 them. In these changes, and generally in all those 

 which the atmosphere undergoes, the sum of the pro- 

 ducts of the particles of the body, and of its atmo- 

 sphere, multiplied respectively by the area which 

 their radii vectores projected on the plane of the equa- 

 tor describe round their common centre of gravity, 

 are always equal in equal times. 



Supposing then, that by any cause whatever, the 

 atmosphere should contract itself, or that a part 

 should condense itself on the surface of the body, the 

 rotatory motion of the body, and of its atmosphere, 

 would be accelerated, because the radii vectores of 

 the area, described by the particles of the primitive 

 atmosphere becoming smaller, the sum of the pro- 

 duct of all the particles, by the corresponding area, 

 could not remain the same, unless the velocity of ro- 

 tation augments. 



At its surface the atmosphere is only retained by 

 its weight, and the form of this surface is such, that 

 the force which results from the centrifugal and at- 

 tractive forces of the body, is perpendicular to it. 

 The atmosphere is flattened towards the poles, and 

 distended at its equator ; but this ellipticity has li- 

 mits, and in the case where it is the greatest, the 

 proportion of the axis of the pole and the. equator is 

 as two to three. 



The atmosphere can only extend itself at the equa- 

 tor, to that point where the centrifugal force exactly 

 balances the force of gravity ; for it is evident, that 

 beyond this limit the fluid would dissipate itself. 

 Relative to the sun, this point is distant from its 

 centre by the length of the radius of the orbit of a 

 planet, the period of whose revolution is equal to that 

 of the sun's rotation. 



The sun's atmosphere then docs not extend so far 

 as Mercury, and consequently does not produce the 

 zodiacal light, which appears to extend even to the 

 terrestrial orbit. Besides, this atmosphere, the axis 

 of whose poles should be at least two-thirds of that 

 of the equator, is very far from having the lenticular 

 form which observation assigns to the zodiacal light. 



The point where the centrifugal force balances 

 gravity, is so much nearer to the body, in proportion 

 as its rotatory motion is more rapid. Supposing that 

 the atmosphere extends itself as far as this limit, and 

 that afterwards it contracts and condenses itself from 

 the effect of cold at the surface of the body, the ro- 

 tatory motion would become more and more rapid, 

 and the farthest limit of the atmosphere would ap- 

 proach continually to its centre : it will then abandon 

 successively in the plane of its equator, fluid zones, 

 which will continue to circulate round the body, be- 

 cause their centrifugal force is equal to their gravity. 

 But this equality not existing relative to those par- 

 ticles of the atmosphere, distant 'from the equator, 

 they will continue to adhere to it. It is probable 

 that the rings of Saturn are similar zones, abandoned 

 by its atmosphere. 



TOL. III. PART I. 



1 A T N 



If other bodies circulate round that which has Atmu- 

 Been considered, or if it circulates itself round another 6 P h <j re * 

 body, the limit of its atmosphere is that point where ^trlali. 

 its centrifugal force, plus the attraction of the extra- 

 neous bodies, balances exactly its gravity. Thus the 

 limit of the moon's atmosphere, is the point where 

 the centrifugal force due to its rotatory motion, plus 

 the attractive force of the earth, is in equilibrium 

 with the attraction of this satellite. The mass of the 

 moon being y-J-y of that of the earth, this point is 

 distant from the centre of the moon, about the ninth 

 part of the distance from the moon to the earth. If, 

 at this distance, the primitive atmosphere of the 

 moon had not been deprived of its elasticity, it would 

 have been carried towards the earth, which might 

 have retained it. This is perhaps the cause why this 

 atmosphere is so little perceptible." 



The earth's atmosphere must experience similar Oscillations 

 oscillations to those of the ocean, from the action of . at " 



the sun and moon. In an atmosphere, however, like " 

 ours, which is so much agitated by other causes, the 

 winds and variations in the barometer, which, arising 

 from the same cause, have the same periods as the 

 tides, must be very inconsiderable. The change in the 

 altitude of the mercury in the barometer is only about 

 ^5j- of an inch at the equator, where it is a maximum ; 

 though it is not improbable, that the oscillations of 

 the atmosphere, like those of the ocean, may be in- 

 creased by local circumstances. " If we consider all 

 the causes," says La Place, " which disturb the equi- 

 librium of the atmosphere ; its great mobility arising 

 from its fluidity and elasticity ; the influence of 

 heat and cold on its elasticity ; the great mass of 

 vapour that it alternately absorbs and deposes ; and 

 lastly, the changes which the rotation of the earth 

 produces in the relative velocities of its particles, 

 which for this reason are displaced in the direction of 

 the meridians ; we should not be surprised at the in- 

 constancy and variety of its motions, which it would 

 be very difficult to subject to any fixed and certain 

 laws." See Anemometer, Astronomy, Barome- 

 ter, Chemistry, Climate, Meteorology, Pneu- 

 matics, and Thermometer, (o) 



ATMOSPHERES of the Sun, Moon, and Pla- 

 nets. See Astronomy. 



ATMOSPHERES of Electrical Bodies. See E- 

 lectricity. 



ATMOSPHERICAL Clock, the name of a 

 machine proposed by Dr Brewster for measuring the 

 mean temperature of the atmosphere during any given 

 interval. This machine records every variation of 

 temperature that takes place during a given period, 

 and the result indicated on the dial-plate is the exact 

 average of all the heights of the mercury in the ther- 

 mometer. The variations of heat and cold affect the 

 pendulum, which may be either of the tubular or grid- 

 iron kind ; and which is so constructed as to render sen- 

 sible in the motion of the clock the alternate contrac- 

 tions and dilatations which it undergoes. This in- 

 strument shall be fully described in a subsequent part 

 of the work, (o) 



ATNAH, a tribe of Indians, who inhabit that 

 part of the north-west of America, which lies in W. 

 Long. 122, and N. Lat. 52. Their language, ac- 

 cording to Mr Mackenzie, ha* no affinity with any 



H 





