and the Psychrometer. 



441 



vantage in the detection of dew, and in still air this should be 

 raised by a fan. If there is a high wind it will be necessary to 

 put the apparatus in a box, having one of its vertical sides 

 removed, this open side should be placed to the leeward so 

 that the wind will be cut off from the plate. 



When in good condition this apparatus works very nicely 

 and seems to follow fluctuations in the dew-point very closely. 

 The following are taken, from a number of records illustrating 

 this point. 



Fluctuations of the dew-point. 



Deerfield, Mass., 









Oakland, Md., 





Nov. 14, 1884. 





Mar. 



25, 1885. 





Mar. 26. 



21-0° 



12-0° 



11-1° 



12-5° 



22-6° 



19-0° 



20-0 • 



11-6 



11-6 



12-2 



22-7 



19'4 



19-4 



11-4 



12-3 



10-4 



23-0 



20-0 



19-6 



10-0 



11-4 



10-4 



23-6 



20-7 



19-2 



10-2 



11-2 



10-5 



23'3 



21*1. 



18-3 



10-4 



10-9 



11-0 



23-3 



21-6 



19-0 



9-4 



9-0 



11-4 



24-7 



22-2 



190 



10-0 



8-7 



11-5 



25-1 



21-2 



19-6 



9'8 



9-7 



11 -6 



23'6 



20-2 



19-7 



9-1 



9-7 



11-8 



23-6 





20-4 



9-6 



11-0 





22-6 





20-7 



10-3 



10-9 







• 



20-8 



11-1 

 11-0 











These dew-points were determined one after the other as 

 rapidly as possible, the interval being not more than 15 seconds 

 between each reading. There seems to be an uniform change 

 in these readings, sometimes the dew-point increases and then 

 diminishes; as all the conditions were constant during each set 

 it would seem that we have represented an actual fluctuation in 

 the moisture contents of the air. 



Still another form of this apparatus is that devised by Crova. 

 The essential characteristic of this is the deposition of dew on 

 the inside of a hollow, horizontal cylinder, the outside of which 

 is cooled in the usual way. The dew is detected by looking 

 through an eye-piece at one end of the cylinder, and the ap- 

 pearance is said to be highly satisfactory. A most serious 

 objection to this form is the extreme difficulty of properly 

 cleaning and polishing the inside of the cylinder ; another is the 

 impossibility of attaining very accurately the temperature of- 

 the surface on which the deposition takes place. Some remark- 

 able results have been obtained with this apparatus and have 

 been published in the Zeitschrift fiir Meteorologie, 1884, p. 46. 

 In the following table there are given : 1st, the result out of each 

 set of three which showed the greatest difference between Reg- 

 Air. Jour. Sci— Third Series, Vol. XXX, No. 180.— Dec, 1885. 

 28 



