MEASUREMENTS 



329 



(2) 



290-2 x F X e 

 273 4- t ' 



254. The pyschrometer is in common use, but it is a trouble- 

 some and incorrect instrument. Its use will be explained in 

 order that its only useful aspect may be seen. 



It consists of two thermometers, the one dry, the other 

 moistened by a piece of rag, which covers its bulb and receives 

 water from a cotton wick immersed in a tube. The evaporation 

 of this water cools the reservoir and brings it to a temperate 6 

 lower than that of the atmosphere t (fig. 232). 



To find the tension F' corresponding to a temperature 6, the 

 atmospheric pressure H is noted. The tension / will be : 



/=F' AH (* 6). 



A being the constant of the psychrometer, which is determined 

 by a hygrometer ; the latter giving / the equation will be written : 



A F/ ~/ 

 -H(*-6)' 



It is unnecessary to give the principle of this instrument, a 

 principle which is marred by a great lack of precision. But let 

 it be said that the value of A is far from being constant as it 

 varies with the velocity of the wind, the radiation from the walls 

 of the tubes etc., and the place of the observation itself. The 

 psychometer cannot be used below OC. 



If the instrument is rapidly moved resulting in a high air 

 velocity, we can write in general terms : 



A = 0-00082 for above 0C. 



A = 0-00069 for lower than 0C. 



Example : The dry bulb thermometer shows 

 20 or t = 20C. The wet bulb thermometer 

 shows 16, and H = 758 millimetres. Assume that 

 A = 0-00082. Since at 16C, = 16 and F, = 

 13-63, we have: 



/ = 13-63 0-00082 X 758 X 4 = lM6mm. 



The maximum tension corresponding to 20C 

 is 17-52 mm. Therefore the hygrometric state is: 



11-16 



17-52 



0-637. 



255. Friction. A few indications concerning 

 the co-efficients of friction appear to be necessary ( 39). Only a 

 few rather old values are available. 



