MR. W. N. SHAW ON HYGROMETRIC METHODS. 



127 



bulb was read as it stood at rest, and also after it had been swinging pendulum 

 fashion at the end of a string. The mean of differences observed between the 

 stationary and vibrating readings was 0'59. It is difficult to follow the results of the 

 75 experiments included in the table, in consequence of the unusual units employed. 

 The results are, however, more widely divergent than REGNAULT'S. REGNAULT'S 

 constant A is calculated for each observation, and its value varies between '00217 and 

 00094. 



The errors from the true percentage humidity, as given by the chemical method, lie 

 between + 2 and -f- 13 for the swinging instrument, their mean value being 7 '4. 



The subjoined table for computing the dew-point from psychrometer observations is 

 extracted from a letter from Lieutenant NOBLE, of Toronto, published in the ' Pro- 

 ceedings of the Royal Society,' vol. 7, 1855, p. 528. It was obtained by means of 

 simultaneous observations with REGNAULT'S hygrometer. The factor,/, of the second 

 column, corresponding to the Greenwich factor,* is that by which the difference of 

 tempemtures of the wet and dry bulb must be multiplied in order to give the difference 

 between the air temperature and the dew-point, the formula being T = t f(t t'). 



TABLE of Factors by NOBLE and CAMPBELL. 



This method of reducing results is quite different from those already mentioned, 

 and is therefore only comparable with them by taking some actual observations and 



See Note B., p. 147. 



