Humidity of the Air, 173 



F = Glaisher's factor at dry bulb temperature 

 d = Difference between dry and wet. 



The arrangement of the table of curves is as follows : — 61 

 equidistant vertical lines were drawn to represent dry bulb tem- 

 peratures from 30 to 90 ; on a sufficient number of these lines, 

 and starting always from the straight line marked 100, with a 

 scale of equal parts called Index. I marked the difference 

 between the dry and wet bulbs for all degrees of humidity, from 

 40 to 99 ; lines were then drawn through these points and con- 

 stitute the curves. 



The vertical lines may be taken to represent so many ther- 

 mometers with equal scales arranged in reference to lines 100, 

 and starting from 30", each one degree lower than the preceding 

 one ; upon the lower parts of these scales were marked the 

 reading of the wet bulb for all degrees of humidity from 40* to 

 99' under each dry bulb reading. 



From this it is evident that given any dry bulb temperature 

 and the difference between it and the wet, it is only necessary to 

 place the index on the dry bulb line, look down it till the 

 difference is found, and opposite will be found a line indicating 

 the required humidity. 



Before closing these remarks there is one point to which I 

 should like to draw your attention. I have drawn the curve 

 representing 40' of humidity on an enlarged scale bearing in 

 mind then the expansion of water below 40" it is curious that 

 this curve, so regular from 100' to 40', should from 40* to 26' 

 take such a remarkable change in direction, and then from 26* 

 go on again almost parallel to its first part. I have not been 

 able to see any satisfactory explanation of this, but I should 

 have expected a regular curve. 



