o4 Mr. Nixon on the [Jan*. 



divide the neutral point (31 inches) by 30 + 1, and deduct the 

 capillarity (O'lUO) from the quotient (I'OOO). The scale being 

 elevated by the remainder (0'900), the observed heights will be 

 proper for calculation without correction for capacity or capil- 

 larity. When the alteration cannot be made conveniently, 

 deduct from the observed pressures a constant quantity found as 

 just indicated. 



In narrow tubes the mercury will not move so freely as in 

 those of larger diameter. On the other hand, as the latter are 

 more liable to be broken, it will be advisable to make choice of 

 one with a capacity of about l-50th or l-60th. 



The thermometer, divided into single degrees of Fahrenheit's 

 scale, should be well graduated, having its fixed points accu- 

 rately laid down. Should the thread of mercury ever appear 

 divided, the instrument may be rejected as unserviceable. la 

 observing the height of the tliermometer, the eye must be held 

 perpendicular to the summit of the column, and at a moderate 

 distance from it. 



A light tripod, similar to that of a theodolite, on which to 

 suspend the barometer, is absolutely indispensable. To the 

 tripod should be added as an occasional substitute a rod of iron, 

 nearly of the diameter of the suspension ring of the barometer, 

 and about 15 inches in length. It should be turned up a little 

 atone end, and flattened and pointed at the other, to admit of 

 its being driven into the seams of rocks, walls. Sec. On many 

 occasions a 12-inch telescope, fitted up with adjustable cross- 

 wires and ground spirit level, will be found to be extremely 

 useful. 



In possession of the requisite instruments, the- observer will 

 naturally inquire, what are the most favourable circumstances as 

 to pressure, temperature, &.c. under which the observations may 

 be made ? 



In the first place, as the pressures observed at the base of the 

 mountain before and after the ascent, serve, together with the 

 elapsed time, as data whence to obtain by interpolation the 

 pressure at the time of the'observation on the summit, the most 

 favourable period as to pressure, will be when the barometer is 

 stationary, or nearly so, and at its mean height for the place.* 

 In this favourable condition of the atmosphere, the pressures at 

 equal altitudes will most probably be equal over a considerable 

 extent of country, and the variations at the base in the interval 

 of the two observations insignificant, and what is of more 

 importance, and may- rarely occur in opposite circumstances, 

 sensibly uniform. The horizontal distance of the stations being 

 inconsiderable, a maximum pressure may be preferable, the 



• To the pressure expressed in thousandth parts of an inch, add the height in feet of 

 the place of observation above the sea, which will give the pressure at that level wi h 

 sufficient exactness. The mean pressure at the sea is considered to be 30000 inchc* 



