If there were two observers, they would of course work simultaneously, 

 one in the valley and the other on the hilltop, to guard against such ma- 

 terial change of temperature or pressure as might take place while a single 

 traveller was climbing. 



Example II. — If the traveller desire to know his elevation above the 

 sea and has no simultaneous observations to refer to, he must assume the 

 pressure at the sea-level to be at its average, viz. 30 inches, and make the 

 best guess he can at what would be the temperature of the air ; the tem- 

 perature decreasing, on a rough average, 1° for each 300 feet elevation. 

 Let us suppose it in the above case to be 52° ; then the height of M x above 

 the sea is thus worked out : — 



15 = 52 



40 W— 64 28 

 A m — 0 = 9560 W=92 and 900 "~ 900 



Uei ht f JVT *) | 

 above sea J 9 ^ 9560 x||= 11 X 28 about = 308 



The following tables refer to S, and explain themselves. They are 

 the only ones that have to be applied to an aneroid or to a sympieso- 

 meter : — 



Table I. 



Table II. 



Correction for the difference of 

 gravity in various latitudes. Cor- 

 rection positive from lat. 0° to 

 45°, negative from 45° to 90°. 



Table for decrease of gravity in a verti- 

 cal, acting on the density of the air — 

 always additive. 



Approxi- 

 mate dif- 

 ference of 

 level. 



Latitudes. 



Approxi- 

 mate dif- 

 ference of 

 level. 



Height of Aneroid at Lower 

 Station. 



o 



0 

 90 



10 



80 



o 



20 

 70 



o 



30 

 60 



o 



40 

 50 



inches. 

 16 



inches. 

 20 



inches. 

 24 



inches. 

 28 



feet. 

 16,000 



feet. 

 10,000 



feet. 

 6,000 



feet. 

 2,000 

















feet. 



feet. 



feet. 



feet. 



1,000... 



3 



2 



2 



1 



1 



1,000... 



2 



1 



1 



0 



5,000... 



13 



12 



10 



7 



2 



5,000... 



8 



5 



3 



1 



10,000... 



26 



24 



20 



13 



5 



10,000... 



16 



10 



6 



2 



15,000... 



39 



37 



30 



20 



7 



15,000... 



24 



15 



8 



3 



20,000... 



52 



49 



40 



26 



9 



20,000... 



31 



20 



11 



3 



The aneroid may be briefly described as follows : — 



The weight of a column of air, which in the ordinary barometer acts on 

 the mercury, presses in the aneroid on a small round German-silver box, 

 both surfaces of which are corrugated in concentric circles, to improve the 

 elasticity of the metal. The indications are thus obtained without the use 

 of mercury or any other fluid. 



The new aneroid, which has been so favourably received and extensively 

 patronized by members of the Alpine Club, is the result of a series of ex- 

 periments carried on continuously for several years. The first point ac- 

 complished was the perfect compensation of the instrument for tempera- 

 ture. This was effected by introducing a compound bar of steel and brass 



