1876.] H. F. Blanford — Experiments with the Psychrometer. 63 



the solar eclipse ; the sky was cloudy ; and in the shed, the air was appre- 

 ciably motionless. The shed was screened from such little wind as stirred 

 the anemometer, partly by a neighbouring building, and partly by rising 

 ground. Even on the top of a neighbouring rock, on which the anemome- 

 ter was fixed on a post, there was sufficient air only to stir the vanes at 

 intervals, and in the shed the atmosphere was still and oppressive. Hence 

 it would appear that under all conditions a still atmosphere is unfavourable 

 to the accuracy of the psychrometric method. This conclusion has already 

 been drawn by previous observers. 



These conclusions must nevertheless be regarded as provisional only, 

 until similar experiments shall have been made with more consistent results. 



I have said that the experience now gained has shewn the necessity 

 for some additional precautions and improvements in the use of the Reg- 

 nault's hygrometer in a very dry atmosphere ; and I will mention these, in 

 the hope that other observers may be induced to pursue the investigation. 

 These are — 



1st. — The silver capsule must be highly burnished and free from micros- 

 copic scratches, which so reflect the light as to make it very difficult to 

 seize the moment of dew-deposition. This surface can be given only by 

 hand-polishing with the softest part of the skin (care being taken that the 

 skin is free from grit) and with the application of a little carefully kept 

 jeweller's rouge. Plate powder, wash leather, and soft rag are equally to be 

 avoided ; and when once polished, great care must be taken to preserve the 

 instrument from grit and dust. The Indian rubber tube which communi- 

 cates with the air pipe, must be kept apart from the instrument when not 

 in use, or the sulphur evaporating from it will quickly blacken the surface. 



2nd. — The mouth tube of Casella's instruments should be replaced by 

 a finger bellows or some other portable form of blower which can be worked 

 rapidly but is under complete control ; or else the instrument should be 

 adapted for the use of an aspirator. 



3rd. — A black screen should be so placed at the side of the instrument 

 that its reflection may be thrown from one side of the silver capsule to the 

 eye. 



4ith. — -In experimenting in the open air, great care must be taken not 

 to sit to windward of the hygrometer and not to approach it nearer than 

 is absolutely necessary to take the reading. The mouth and nose should be 

 covered with a handkerchief during the experiment to prevent any breath 

 reaching the instrument. 



5th. — In very dry states of the atmosphere it is best to take a prelimi- 

 nary observation in which the blowing is continued until a decided and 

 comparatively copious deposit is formed, noting as nearly as possible the 

 temperature at which it first appears. This observation is made for guid- 



