18e35.] Meteorological Observations, 5S 



and 10 p. M. as in the case of the thermometer. Having found 

 ^ this opinion borne out by some experiments I made in Egypt, 

 and at Madras I have adopted these two hours for the observa- 

 tions of the hygrometer, as well as of the other instruments ; 

 and even should this method be found, hereafter, not to cor- 

 respond with perfect accuracy to the true average, it will, at 

 all events, in the mean time, ensure uniformity. 



The Rain Gage or Ombrometer, for measuring the quan- 

 tity of rain that falls on any spot, is a well known instru- 

 ment in this country,'and requires no description. 



The Atmometer, or Evaporometer, is an instrument for 

 measuring the quantity of water evaporated in any given 

 time. Its most simple form is that of a round or square 

 vessel of tin, of uniform dimensions, from top to bottom ; 

 the depth of water, evaporated from which, may be easily 

 measured. But a more convenient and more accurate ins- 

 trument is the Atmometer of Leslie, which I shall describe 

 in his own words. " This instrument is an useful auxiliary, 

 and might with «ome attention, serve as a substitute for the 

 hygrometer. It does not mark the dryness of the air, but it 

 measures, (as its name denotes) the quantity of moisture ex- 

 haled from a humid surface in a given time. The Atmome- 

 ter consists of a thin ball of porous earthenware, two or three 

 inches in diameter, with a small neck, to which is firmly ce- 

 mented a long and rather wide glass tube, bearing divisions 

 each of them corresponding to an internal annular section, 

 equal to a film of liquid, that would cover the outer surface 

 of the ball to the thickness of the thousandth part of an inch. 

 The divisions are ascertained by a simple calculation, and 

 numbered downwards to the extent of 100 to 200 ; to the 

 top of the tube is fitted a brass cap, having a collar of lea- 

 ther, and which, after the cavity has been filled with dis- 

 tilled water, is screwed tight. The outside of the ball being 

 now wiped dry, the instrument is suspended out of doors, and 

 exposed to the free access of the air. In this state of action 

 the humidity transudes through the porous substance just as 

 fast as it evaporates from the external surface, and this waste 

 is measured by the corresponding descent of the water in the 

 stem.'* 



" The dissipation of moisture is much accelerated by the 

 agency of sweeping winds^ the effect being sometimes aug- 



