172 Humidity of the Air. 



Taking the same example as before, and working out the 

 humidity by these tables, we get — 



Humidity opposite wet bulb in tables ... 59 

 For an increase of l - in wet bulb+ 4 x .7= + 2.8 



61.8 

 For an increase of 1" in dry bulb — 4i.3= — 1.2 



60.6 



It will be seen that Mr. Grlaisher's tables save a great deal of 

 time ; but, simple as the operation is, when it has to be often 

 repeated, there is a risk of error from the use of opposite signs. 



I have for some years felt convinced that it was possible so to 

 arrange a table of humidity as to avoid the necessity for calcula- 

 tion ; but whenever I have tried to arrange it in figures, the 

 extent of the table appeared to stand in the way of its usefulness. 

 I once calculated all the useful values of F -4- N from 26" to 81* 

 for Apjohn's formula, and the results alone occupy 120 pages of 

 closely- written figures. 



It was not until some few months since, when considering the 

 matter, that I saw how to construct these curves, which I think 

 have accomplished the desired result. 



For thirteen years a regular series of observations were taken 

 at the Eoyal Observatory, Greenwich, with dry and wet bulbs 

 and direct dew point instruments (Daniel's) with a view to obtain 

 the relation between them. The result is given in the form of a 

 table in the Greenwich results, and also in Mr. Glaisher's tables, 

 of which it forms the basis. This table gives factors by which 

 to multiply the difference between the dry and wet bulbs in 

 order to obtain the difference between the dry bulb and dew 

 point. 



From this table I have computed the differences between the 

 dry and wet bulbs corresponding to all degrees of humidity from 

 40 to 99, by means of the formula. 

 T . h 



=T' 



100 



T' in tables of tension of vapour gives P 

 then B— P 



F 

 where 



T = Tension of vapour at dry bulb 

 h = Humidity 



T' = Tension of vapour at dew point 

 P = Dew Point 

 B = Dry bulb 



