134 MR \V. N. SHAW ON HYGROMETRIC MP]THOUS. 



He therefore assumed that A was a function of t t', and, writing 



A = a + b (t t'), 



determined the values of a and 6 for each series by CAUCHY'S method, giving to each 

 equation a weight equal to the number of observations from which the values A and 

 t t' were obtained. 



The 36 values of b thus obtained presented no regular variation, whether arranged 

 with reference to t t', t', or h. 



He therefore supposed b to be constant and found it equal to 0'000028. 



The values of a increased when t', and consequently f, increased, and when h 

 diminished and could be represented by the equation 



a= 0-000776 + 



n 



The psychrometric formula therefore became 



x = /' [1 - 0-0159 (t - t')~\ - 0-000776 h (t - t') [l - 0'0361 (t - t')] 

 for temperatures above C., and 



='[! 0-0159 (t t')] 0-000682 h (t t') [1 0'0411 (t - t')] 



for temperatures below C. 



From these formulae tables have been constructed ; these are not given in the 

 abstract in the ' Journal de Physique,' but a diagram which serves the purpose is 

 appended at the end of the paper. 



The results with these tables seem to be very satisfactory, as compared with those 

 given by HAEGHENS'S tables, the mean percentage error of 90 observations at Paris 

 being -f- 1*1 by the latter and only 0'5 by the former; while for 91 observations of 

 CHISTONI'S the mean error by the old tables is 2'5, and by ANGOT'S only + 0'5 ; and 

 further, applying the new tables to REGNAULT'S observations below the freezing point, 

 the mean error is only + 0'7, as compared with +3*5 obtained when REGNAULT'S 

 formula is employed. On the whole ANGOT states that the error of the computation 

 from a single observation above zero is about i 2 units in the relative humidity, and 

 the absolute mean error of a series of some 20 to 30 observations 0'5 ; for tempera- 

 tures below zero it would probably be necessary to double these figures, but at any 

 rate even for very hot and dry regions the tables never give negative values for the 

 humidity, which is a possibility with the old tables. No particulars are given as to 

 the mounting and moistening of the thermometers. 



An account of an interesting series of observations with the psychrometer in very 

 dry atmosphere is given by H. F. BLANFORD in the ' Journal of the Asiatic Society 

 of Bengal,' vol. 45, 1876. The observations were taken in various stations in India, 



