244 On the Use of the comrnon Titennnmeter as a Hxjgrometer, 



divieied into a thousand equal parts. If one of the balls be co- 

 vered witli cambric or silk, and wetted with pure water, the in- 

 r,tnuuent Forms a complete hygrometer ; for it will mark, by tlie 

 descent of tlie column in the opposite .■stem, the constant dimi- 

 nution of temperature which is ciuisod by evaporation from that 

 hun/id surfice, and it must couseciiiently cxjhcss the relative dry- 

 !iess of the ambient air," It is hardly necessary to observe, that 

 hydrometers constructed on this priucijjle must always indicate 

 the same dryness, in the same circumstances, and may therefore 

 be as readily con)pared with one another as thermometers them- 

 selves. But my obJL^ct is not so much to di'^cuss the merits of 

 the instrument itself, as to show that the common therujometer 

 maybe used in its stead; and that though it may not possess the 

 s ime degree of delicacy, it is sufficiently accurate for all the or- 

 dinary purposes of meteorology. Let two spirit-of wine ther- 

 mometers be chosen, as nearly of the same size as possible, and 

 graduated so as exactly to coincide at different temperature^. 

 Let the bulb of one of them be covered with blue or purple silk 

 while the other remains naked, and let them be suspended at 

 about the distance of two inches from each other. Let the co- 

 vered bulb be then wetted with pure water, and the two thermo- 

 meters will very soon indicate a difference of temperature; the 

 wetted one, from the cold produced by the evaporation, sinking ^ 

 below the other, more or less, according to the rapidity of the 

 evaporation ; that is, according as the air is more or less dry. 

 If the thermometers be graduated according to Fahrei^iheit's 

 scale, each degree of difference must be multiplied by 54, and 

 the product will express the degrees of the Professor's hygrome- 

 ter nearly; or if they are graduated according to the centigrade, 

 scale, the degrees of difference, multi|)lied by 10, will give the, 

 hvgrometric degrees exactly. From numerous comparative oIit 

 servations, I am able to say, that the average dryness of a month, 

 as indicated by the thermometers, will not differ from that in- 

 dicated by the' hygrometer more than two hygrometric degrees, 

 a quantity that mav be safely overlooked in a series of oliserva- 

 tions which do not admit pf extreme accuracy. It may perhaps 

 look like presumption, but I cannot help observing, that the 

 thermometers apj)ear to me better calculated to give the mean 

 dryness of the air than the hygrometer itself, as the latter, from 

 its" extreme delic.icv, is sometimes aH'ected by a sudden gust of 

 wind at the moment of observation, so as to rise two or three, 

 degrees. There is, however, one obvious advantage which thq 

 thermometers possess over the hygrometer, and that is, thgir 

 showing not onlv the difference between the temperatures of 

 the two bulbs, which is all that the hygrometer shows, but also 

 the actual temperature of both the wet and dry surface, a cir- 

 cumstance 



