IOC Mr H. Meikle on finding the Dew-pointy <^c, 



B, the barometric pressure. In this Journal for December 1826, 

 I had proposed to multiply D, the depression of the wet ther- 



mometer, by — ^ — ; from observing that, when the tempera- 

 ture of the air was constant, this reduced the different depres- 

 sions to one value, or made them likewise as if constant; and 



therefore it was obvious that, by making t z=it —^ D, the 



several cases became the same as if the pressure too had been 

 constant. There can be Httle doubt that this was nearly cor- 

 rect, for the particular series of experiments from which it wa.* 

 deduced, though perhaps for it alone, rather than cases in which 

 the air is more humid. But when we alter the value of ^, the 

 whole concern is unhinged, not one of the quantities continues 

 constant throughout that series of experiments, and we have no 



evidence that the multiplier — ~^ is at all near the truth. 



That gentleman's view of the matter, I readily grant, would 

 have been quite correct with respect to that series, had it been 

 the temperature of the moist bulb, and not of the air, that was 

 constant ; but then a much greater correction would have been 

 required, as 1 find the following experiments, where t is con- 

 stant, would give ^ "*" ^ for the multiplier. 



B t 



30 46M 



10 59 .9 



It is farther to be observed, that the actual temperature of 

 the air is much higher in the second of these cases than in the 

 first ; and it appears from Professor DanielPs experiments, that 

 when two equal vessels, the one containing water and the other 

 sulphuric acid, were put under a receiver on the plate of an air- 

 pump, at the temperature of 52°.3, the dew-point was 35°. 7, but 

 when the temperature was 60°. 7, the dew-point was no lower 

 than 48°.4 *. Hence there is reason to suspect, that the air at 



• Meteorological Essays^ 2d edit. p. 498. There were seven experiments 

 of each sort, of which I have taken the mean ; and, in seeking for the mode 

 in which the dew-point varies with the temperature in the given circum- 

 stances, I find that the evaporating force (as Mr Dalton would call it, though 

 perhaps he would only name it so in air of the ordinary density), is the same 



