MB. W. N. SHAW ON HYQROMETRIC METHODS. 147 



BLANFORD'S tables are bused upon the same observations and formulae, but they are 

 adapted for English inches and FAHRENHEIT degrees (see p. 135). The following 

 information respecting them is extracted from the introduction to the tables (p. 4) : 



""TABLE III. This table gives the tension of saturated aqueous vapour, in decimals 

 of an inch of mercury at the temperature 32, in latitude 22, at the level of the sea*. 

 It has been reduced from the original table for the latitude of Dublin, computed (from 

 REONAULT'S observations) by the Rev. ROBERT DIXON, by correcting his values for 

 the difference of gravity, viz., multiplying them by the constant factor 1 '00286184. 



"The psychrometric tables which follow are all based on this table, and the 

 computation has been chiefly made by the aid of the arithmometer. 



" AUGUST'S formula (modified by REGNAULT), which has been used in computing 

 the Tables IV. to XL, is as follows :-r- 



" For temperatures of the wet bulb below 32 : 



-430 (<-Q 

 ~ J " 1240-2 -f 



and for temperatures of wet bulb above 32 : 



-480 (t-f) 



X =S - 1130 _ ,' h - 



wherein t' and t are the temperatures of the dry and wet bulb thermometers respec- 

 tively, in FAHRENHEIT degrees, f the tension of vapour at temperature t', h the reading 

 of the barometer in inches, and x the tension of the vapour present in the air at the 

 time of the observation." 



GLAISHER'S tables are based upon REGNAULT'S table of pressures, and the table of 

 Greenwich factors, which the following extract from the introduction to GLAISHER'S 

 tables (p. 4) sufficiently describes. These are adapted for English inches and FAHREN- 

 HEIT'S scale of temperature : 



"Determination of the Dew-point from, obsemations of tJie Dry and Wet Bulb 



Thermometers. 



" TABLE I. Factors by which it is necessary to multiply the excess of the reading 

 of the dry thermometer over that of the wet, to give the excess of the temperature 

 of the air above that of the dew-point, for every degree of air temperature, from 10 

 to 100. 



" The numbers in this table have been found from the combination of many 

 thousand simultaneous observations of the dry and wet bulb thermometers and of 

 DANIELL'S hygrometer, taken at the Royal Observatory, Greenwich, from the year 

 1841 to 1854, and from observations taken at high temperatures in India, and others 

 at low and medium temperatures at Toronto. The results at the same temperatures 



u 2 



