2G0 Dr. Hudson's Method fur inferring the Dew-point 



portion) what number of degrees dry air should fall at any 

 other temperature of the hygrometer on the supposition that 

 air in sinking a given number of degrees (at those different 

 temperatures) gives out the same quantity of heat in each 

 case, from a given volume. If this be not so, we must allow 

 for it in the manner to be hereafter mentioned. To try the 

 amount of effect of dry air at different temperatures on wet- 

 bulb hygrometer, I would propose to pass air (well dried by 

 muriate of lime) from a gasometer through a tube which could 

 be heated to different degrees at pleasure, and in the con- 

 tinuation of this tube (made of glass) to suspend a thermome- 

 ter and wet-ball hygrometer, so as to be equally exposed to 

 current of air, and note the temperature of each (in a tabular 

 form) as the temperature of air varied. The current should 

 be rapid, and the source of heat large, so that temperature 

 should alter slowly. There should be various precautions 

 adopted, which I shall not dwell on here. 



3tio. In order to ascertain the value (in degrees of Fahren- 

 heit) of the proportional numbers, we may evidently use the 

 formula " V : V — d : : moisture of saturation at tempera- 

 ture of hygrometer : moisture of saturation at dew-point," by 

 conjoint observations of both hygrometer and dew-point. 

 Thus, suppose (for example) that thermometer marks 100° 

 and hygrometer 61°, and dew-point found by observation 

 = 52 o, 408, then (by inverting former formula) we have 

 "moisture of saturation at 61°; moisture of saturation at 

 52°*408 : moisture of saturation at 61°:: (100-61) 39 : 51°*124 

 = V, the fall in dry air corresponding to temperature 61° of 

 hygrometer. 



Having tried Daniel's hygrometer (with this view), I doubt 

 whether we can attain (through this or similar instru- 

 ments) a knowledge of the actual dew-point without being 

 liable to an error of, perhaps, a degree or more, at the ordi- 

 nary temperatures of the dew-point in these countries. It must 

 be evident from the very small differences of the saturating 

 weights of moisture (in a cubic foot of air) at the ordinary 

 temperatures of the dew-point, where the temperature alters 

 one degree, that the amount deposited in a short time can 

 hardly be perceptible to our senses. For instance, suppose 

 the point of saturation is 58° and that temperature is reduced 

 to 57°, the quantities of saturation (in a cubic foot) are 5*492 

 grains (at 58) and 5*345 grains (at 57), so that a cubic foot of 

 air acting on a given surface, could not, under these circum- 

 stances, deposit 0*15 grains of moisture on that surface (al- 

 though 1° below its point of saturation). Now, unless the 

 surface be very small (in which case the time which a cubic 



