ATM 



[41 ] 



A T 



ASTROITE. See Asterite. A name 

 given by some writers to corals of 

 the genus Astrea. 



ATA'CAMITE. Prismatoidal green mala- 

 chite. Native muriate of copper, 

 of a green colour, occuring both 

 massive and crystallized. It con- 

 sists of oxide of copper 76 "6, 

 muriatic acid 12 '4, water 12. It 

 has obtained its name from having 

 been found in alluvial sand in the 

 river Lipas, in the desert of Atacama 

 in Peru. It has also been found in 

 some of the Vesuvian lavas. The 

 primitive form is an octoedron. It 

 is the Cuivre muriate of Haiiy. 



ATMO'METER. (from ar^o?, vapour, 

 and yM-fc-Tyoe'o), to measure, Gr.) An 

 instrument contrived by Professor 

 Leslie, for ascertaining the quantity 

 of moisture exhaled from a damp 

 surface in a given period. 



ATMOS'PHKRE. (from ar/to?, vapour, 

 and a<paipa, sphere, Gr. atmosphere, 

 Fr. atmosfera, It.) The gaseous 

 compound which surrounds the 

 earth : it is a thin, transparent, 

 invisible, and elastic fluid, essen- 

 tially composed of oxygen and 

 nitrogen, yielding by analysis 79 

 parts of nitrogen to 21 of oxygen, 

 and containing in every 1000 parts 

 three or four of carbonic acid gas. 

 These proportions are found to be 

 the same at all heights hitherto 

 attained by man. The atmosphere, 

 from its powers of refraction, has 

 been calculated to extend upwards 

 about forty-five miles. That the 

 atmosphere is a ponderous bod}^ 

 was first suspected by Galileo, who 

 found lhat a copper ball, in which 

 the air had been condensed, weighed 

 heavier than when the air was in 

 its ordinary state of tension. The 

 fact was afterwards demonstrated 

 by Torricelli, whose attention was 

 drawn to the subject by the attempt 

 of a well digger to raise water by a 

 sucking pump to a height exceeding 

 33 feet. 



The pressure of the atmosphere 

 is about fifteen pounds on every 



square inch ; so that the surface of 

 the whole globe sustains a weight 

 of 11,449,000,000 hundreds of 

 millions of pounds. Shell-fish 

 which have the power of producing 

 a vacuum, adhere to the rocks by a 

 pressure of fifteen pounds upon 

 every square inch of contact. 



When in equilibrio, the atmos- 

 phere is an ellipsoid flattened at the 

 poles, in consequence of its rotation 

 with the earth. 



The barometer, by its fall and rise, 

 indicates a corresponding change in 

 the density of the atmosphere. At 

 the surface of the earth the mean 

 density or pressure is considered 

 equal to the support of a column of 

 quicksilver 30 inches high ; at 1000 

 feet above the surface of the earth 

 the column of quicksilver is found 

 to fall to 28*91 inches; at one mile 

 to 24'67 inches; at two miles to 

 20*29 inches; at three miles to 

 16'68 inches; the density decreas- 

 ing upwards in geometrical pro- 

 gression. The air even on moun- 

 tain tops is sufficient to diminish 

 the intensity of sound, to effect the 

 breathing,- and to produce a loss of 

 muscular power. The greatest 

 elevation attained by man, is 4'36 

 miles; that height M. M. Gay- 

 Lussac and Biot ascended in a 

 balloon, but they suffered severely 

 from the rarity of the atmosphere. 



A'TOLL. The name given to a ring- 

 formed coral reef. 



ATRY'PA. (from a, priv,, and rpwra, 

 a foramen, Gr.) A genus of fossil 

 shells found in the Lower Siluvian 

 Bocks. A sub-division of the 

 family of Terebratula. "This 

 genus is divided from Spirifer, and 

 includes those species which have 

 a short hinge line without a large 

 area, and are either destitute of a 

 foramen or possess only a small 

 triangular one. They are rounded 

 shells, and are not furrowed like 

 the typical species of Spirifer. 

 The species of this sub-division 

 have generally been described as 



