130 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



" Observations Anormales des Psychrom^tres" (' Bulletin de la Socidte Vaudoise,' 

 vol. 9, p. 234) nor the " Rapport de la Commission Hygrome"trique" (' Actes de la Soc. 

 Helvetique des Sciences Naturelles,' 1866). 



CHISTONI, at the request of the Italian Meteorological Office, undertook a compa- 

 rison of the ordinary psychrometer and a ventilation psychrometer with a standard 

 hygrometer. The ventilation psychrometer is an instrument provided with two 

 centrifugal fans on the same axle driven by clockwork. Each fan leads to a separate 

 vertical tube, and the bulbs of the two thermometers are placed in the mouths of 

 these tubes, so that air passes over them at a rate which is sensibly constant, and the 

 same at each observation. The experiments were carried out at Ostiano. REGNAULT'S 

 hygrometer was chosen as the standard instrument with which to compare the psychro- 

 raeters, for the following reasons : (l) The chemical method was too cumbersome for 

 use in the country ; (2) BELLI'S standard hygrometer might be supposed to affect the 

 air in its neighbourhood by its large cold mass. The air was drawn through 

 REGNAULT'S hygrometer by means of a very ingenious aspirator formed by allowing 

 sand to fall from a cloth funnel down a long vertical tube, provided simply with 

 a side opening connected with the hygrometer. 



The observations were made in a school-room with three windows, with two psychro- 

 meters, each used in turn as a ventilated and non-ventilated instrument. They were 

 reduced by the ' Tavole ad uso della Meteorologia.'* 



A summary of the results is given in the following table : 



The constant A of REGNAULT'S formula was calculated for each of the ventilator 

 observations ; its value varies between 0'00072 and 0'00098 for No. 1, and between 

 G'00069 and O'OOllO for No. 2. These results are strong evidence in favour of the 

 ventilation method. 



The wet bulbs were covered with muslin and moistened before each observation, so 

 that the results are not necessarily applicable to cases in which the wet bulb is kept 

 continually moist by means of a wick or other capillary arrangement. 



CHISTONI extended his observations to temperatures of the dry bulb below 15 C. in 

 the winter months of 1877- 8 at Pavia. The summary is as follows : 



* See below, p. 148. 



