102 Mr H. Meikle on finding the Dezv-Point, 



note the force of vapour at the respective temperatures, and al-. 

 most close on the right of that, again, might be placed another 

 curve pretty similar to the hyperbola, and nearly equidistant 

 from it, for the purpose of the ruler marking on it the tempera- 

 ture to which, if the air were cooled without any diminution of 

 volume, it would be just in a state of saturation with moisture. 

 Or, if close on the right side of this new curve, a set of divisions 

 were put, to note at once the density of vapour at the respective 

 temperatures, the labour of finding the actual density, or weight 

 of vapour in the air, would be still farther abridged, or almost 

 avoided. On the left of the other hyperbola PZ, and as it were 

 parallel to it, several curves might be placed, and the more dis- 

 stant, respectively from it, as the barometric pressure was smaller. 

 These are meant to be used in place of PZ, when the pressure 

 falls short of 30 inches. When it exceeds 30, there is scarcely 

 any need for correction for pressure ; but that, too, could easily 

 be provided for by a similar curve on the other side of PZ. 



Were this project realized, that is, if, by merely laying a ruler 

 across a plate, in the manner above mentioned, the force, den- 

 sity, &c. of the vapour in the air, could be indicated at once, 

 and without computation, it would obviously be the most con- 

 venient method yet employed for the purpose, and would su- 

 persede the use of troublesome and expensive instruments for di- 

 rectly observing the dew-point — often with a considerable de- 

 gree of indistinctness and uncertainty. 



The ingenious author of the article above quoted, holds some 

 opinions which differ materially from my own. I cannot, for 

 instance, conceive how the wet bulb could continue to furnish any 

 heat for the formation of vapour, after the process has fairly com- 

 menced. When the instrument is first exposed to the drying 

 influence of the air, evaporation takes place, and lowers the tem- 

 perature of the bulb, by abstracting heat from it for the forma- 

 tion of vapour ; but a limit is soon set to the fall of temperature 

 by the warmer air, which, in successively touching the wet and 

 colder surface, bears a share of the loss of heat, or, in other 

 words, imparts heat by contact to the colder bulb, and also, no 

 doubt, by the warmer surrounding bodies throwing in a very 

 little heat upon it by radiation. The heat thus imparted and 

 thrown in, is, next to all that I can think of, as being s})ent in 



