839 



ATLANTES. 



ATMOSPHERE. 



their heads and arms the entablature on which the beams of the roof 

 were to hare been placed. The Atlantes of this temple were 25 feet 

 high, built in courses of stone, corresponding with the walls of the cella, 

 and partly attached to it. The annexed cut (Fig. 1), showing the front 

 elevation of the figures, with a profile of one of them, has been engraved 

 with the permission of the publishers of Stuart's ' Athens.' (For a 



Yig. 2. Caryatid I'llimtcr at Modinet Abou. 



more detailed account of these figures, see vol. iv. cap. i. of Stuart's 

 ' Athens.') 



In the Tepidarium of the baths at Pompeii, Atlantes of baked clay, 

 in high relief, and incrusted with the finest marble stucco, painted to 

 represent life, are ranged at equal distances round the room, to support 



Fig. 3. From Pompeii. 



between the figures, niches are formed 

 AllTS AMD SCI. DIV. VOL. I. 



hed ceiling springs ; in i 

 onned for the dress of 



the bathers. 



The figures are about two feet high, and stand, like those at Agrigentum, 

 on a plinth ; they are represented in Fig. 3. 



In the architecture of the modern Italians, the Atlantes are often 

 found supporting the entablature over an entrance to a palace or a 

 garden. At Milan, there is a colossal example of the former ; and the 

 rustic gate to the Farnese Gardens at Rome, the design of Vignola, 

 may be adduced as an example of the latter. 



The propriety of employing representations of human figures as 

 supporting the entablature or roof of a building, is at least very 

 questionable. 



ATLAS, a collection of maps ; so called probably in allusion to the 

 mythological figure of Atlas represented as bearing the world upon his 

 shoulders, symbolical of Mount Atlas. 



Boucher in his ' Glossary,' says, the word seems to be derived from 

 the German, in which language atlass means satin ; because a collection 

 of maps was usually made of a smooth satin paper. The word is also 

 now frequently used, and probably for the same reason, for a collection 

 of plates illustrative of a work which is printed on a size much larger 

 than the work itself. It is generally, however, in such cases, distin- 

 guished as an Atlas of plates. 



ATMOSPHERE (from the Greek ctT/ios,and attxupa, sphere of vapour), 

 is the whole body of air or other mixture of gases which envelopes a 

 planet. We shall here devote ourselves exclusively to that which 

 surrounds the earth, merely observing, that we have more or less 

 reason to suppose that atmospheres, in density comparable to that of 

 the earth, envelope the Sun, Venus, Mars, Jupiter, and Saturn, but not 

 the Moon. 



The subject of the atmosphere, treated in all its extent, would lead 

 us much too far ; we shall therefore confine ourselves to the description 

 of its average state. We have already discussed the properties of its 

 constituent materials in the article Am, and we must further refer as 

 follows, both for subjects which we cannot here enter upon, as well as 

 for extensions of various points which must be incidentally mentioned. 



For the general subject of the atmosphere, as connected with tho 

 weather, see METEOROLOGY, HYGROMETRY, TEMPERATURE, and articles 

 on particular subjects, such as EVAPORATION, DEW, RAIN, WIND, 

 AURORA BOREALIS, HEAT, ELECTRICITY (ATMOSPHERIC), &c. &c. 



For the atmosphere as a medium of communication (taking this 

 word in its widest sense), see ACOUSTICS, AERODYNAMICS, BALLOON, 

 WINDMILL, SAIL. For its effects upon animal and vegetable life, see 

 RESPIRATION, VEGETATION, ANTISEPTICS, DECOMPOSITION. For the 

 effects of the imponderable substances upon it, and vice versd, see 

 HEAT, ELECTRICITY, REFRACTION. For instruments used to measure 

 ita state, see BAROMETER, THERMOMETER, MANOMETER, EUDIOMETER, 

 HYGROMETER ; and for its uses hi the investigation of the elevations of 

 different parts of the earth, see BAROMETER, HEIGHTS (MEASURE- 

 MENT OP). 



The atmosphere, in its average state, must be considered as a body 

 of air revolving with the earth. This gives its several strata an in- 

 creasing velocity, as we recede from the earth's axis : and hence, the 

 convex surface of the atmosphere is probably spheroidal, and not 

 spherical, since, in the first place, its height would be greater over the 

 equator by reason of its greater centrifugal force, and secondly, by 

 reason of the expansion of the same parts by the greater temperature. 

 Again, at the equator, the air (if any) which is twice as distant from 

 the centre of the earth as the surface, must revolve with twice the 

 actual velocity of the air at the surface. This consideration shows 

 positively that the atmosphere which really accompanies and revolves 

 with the earth cannot certainly extend in the smallest quantity above 

 26,000 miles from the surface. For at that height the tendency to 

 recede from the centre, known by the name of centrifugal force, would 

 counterbalance the weight, or tendency of particles towards the earth, 

 and at higher distances would overcome it entirely. 



But we are not therefore to conclude that there must be air, more or 

 less, revolving with the earth up to so great a height. Forty or fifty 

 miles is supposed to be the limit which it attains. Previously, however, 

 to entering upon this question, it is material to know whether we are 

 to consider air as infinitely divisible or not. By which we mean, is it 

 possible for air to be rarefied to any extent whatever, and still preserve 

 its great characteristic, namely, mutual repulsion among its several 

 parts 1 We might mention various arguments drawn from the ATOMIC 

 THEORY, but Dr. Wollaston (' Phil. Trans." 1822) has discussed this 

 subject in a form which, while it adds some force to the atomic theory 

 itself, for reasons unconnected with our subject, furnishes a very strong 

 presumption for the finite extent of the atmosphere. The following is 

 a synopsis of his argument : 



If there be air throughout the universe, we are obliged to suppose 

 that every planet would collect an atmosphere around itself, propor- 

 tionate to its attracting power. In this case, we know that Jupiter, at 

 whose surface the force of gravity must be much greater than at that 

 of our earth, would collect a powerful atmosphere around him. The 

 effect of the refraction of light through this atmosphere would become 

 visible in the approach of the satellites to the planet, when they dis- 

 appear behind his disc, and would cause a sensible retardation in their 

 rate of approach. No such retardation can be observed hi the smallest 

 sensible degree ; and, consequently, Jupiter has no such atmosphere, 

 nor the means of collecting it : consequently, air, such as we have at 

 the earth, ia not diffused in any degree of rarefaction through the 



T T 



