124 MR. W. N. SHAW ON HYGROMETRIC METHODS. 



After laying down the following objections to DANIELL'S hygrometer, viz. : 



1. The temperature indicated by the thermometer may be different from that of 



the surface upon which the deposit is formed. 



2. The observer is too close to the instrument. 



3. The evaporation of the ether on the bulb may alter the temperature and hygro- 



metric state of the air. 



4. The ether generally contains some water, which evaporates and changes the 



hygrometric state of the neighbouring air still more. 



5. In dry weather it is generally difficult, and sometimes impossible, to get a deposit 



of dew at all. 



REGNAULT describes his own instrument, proposing to produce the evaporation of the 

 ether in the capsule by causing air to bubble through it by means of an aspirator, 

 which enables one to adjust the temperature with very great nicety. No comparisons 

 of results with those of the chemical method are given. 



The memoir then describes a large number of experiments upon the pyschrometer, 

 the object of which was to determine to what extent AUGUST'S formula 



, _ 0-4290 - Q . 

 J~J 610 -f 



is applicable to psychrometer observations. 



It is first made clear by direct experiment that when perfectly dry air is made to 

 pass over a dry bulb and wet bulb successively the temperature of the latter depends 

 upon the rate at which the air passes ; from this it is inferred that a similar result 

 will be found with air more or less moist, and this is verified by direct experiment. 

 Several sets of experiments, however, including a series upon two wet bulbs exposed 

 in an open court 2 metres from the wall and 7 metres above the ground, one of them 

 being supplied with a current of air of known velocity by means of a ventilating fan, 

 and the other left to itself, lead to the following conclusions : Psychrometer observa- 

 tions in moving air are practically independent of the size of the bulb. The agitation 

 of the air certainly influences the formula for the psychrometer, but when the instru- 

 ment is exposed to free air the same formula can be adopted, so long as the velocity 

 of the wind does not exceed 5 or 6 metres per second. 



REGNAULT suggests the advisability of determining only the form of a reduction 

 formula theoretically, and then assuming constants, determining their values by actual 

 comparison with some other method. 



His psychrometer observations were all reduced to absolute measure by comparison 

 with the results of the chemical method, and in consequence of the comparison the 

 constant of AUGUST'S formula was altered, so that it became 



