223 



THE GEOLOGIST. 



To learn the specific gravity of the column, two main influences have to 

 be taken into account. There are also secondary ones, of so little import- 

 ance that we shall not notice them here. 



(A) The specific gravity of the air, other circumstances being the same, 

 bears a definite ratio to the pressure it sustains ; that is, the greater the 

 pressure, the more it is condensed and the more does a cubic foot of such 

 condensed air weigh. The lower strata of the atmosphere are therefore 

 denser than the higher. 



(B) The specific gravity of the air, other circumstances being the same, 

 is greater in proportion to its coldness. Air that is cold weighs more than 

 air that is hot. 



Formula. — These principles are simple enough. Experiments made 

 with extraordinary care have shown the amount of the above influences, 

 and the laws by which they act. Mathematicians, and notably La Place, 

 have thrown the whole into a formula conveniently adapted for computa- 

 tion. It is substantially as follows, in which the letters A and B refer to 

 the paragraphs (A) and (B), and S represents the secondary considerations, 

 from which we abstain : — 



The height of m' above m — (A m , — A m ) X {1+B-f S}. 



The calculation of the weight of a column of air, reaching to the limits 

 of the atmosphere, on the principle (A), for all pressures of the barometer, 

 at intervals of xooth of an inch, has been made, and forms, in some mo- 

 dified shape, the first of the usual barometric tables. 



Browning's new aneroids, besides being graduated to inches of barome- 

 tric pressure, are also graduated according to the length of the column, de- 

 termined on the principle (A) ; the starting-point having been so taken, 

 that when the barometric pressure is 30 inches, the graduation marks 

 0 feet. The graduation is always made on the assumption of the tempera- 

 ture of the air being 32° Fahr. A m is the graduation that would be indi- 

 cated, under that hypothesis, at the station m, and A m , that at m'. 



To correct these results on the principle (B), the temperature of the air 

 is observed at m and again at m'. The mean of the two temperatures 

 gives at least an approximation to the mean temperature of the entire 

 column of air. The excess of this mean temperature above 32°, divided 

 by 450 (one of the constants that have been determined by experiment), 

 forms B. 



In other words, if t be the temperature at m, and t' at m', 

 E = (£—32°) + (^—32° ) y 1 _. t+t'—Q4P 

 2 450 900 



Hence the first and the important part of La Place's formula is — 



Example I. — Suppose a single traveller to wish to take the height of a 

 mountain, m', above the valley, m. He observes at w, before starting, that 

 his aneroid points, say to 450 feet, and his thermometer to 50° Fahr. On 

 reaching the top of the mountain m 1 , he finds his aneroid to give 9560 feet, 

 and his thermometer to be 40° Fahr. 



t =50 



thenA w ,= 9560 fc40 a , 9f . 



A. =_450 t +¥=W5 } and *-±|=^ = ^ 

 A m ,^A wt =9110 , 900 900 



True height of 7 r§r • 26 1 



m' above m. j 9370 91 10 X §00 = 260 



