471 



arrangement are, that it is less troublesome, by not requiring calculation, 

 reference to tables, or the aid of a barometer, and that it is more sensible 

 to small changes. 



I conceive that my modification of Regnault's instrument is not 

 without advantages : it is more conveniently and generally applicable 

 to its uses, more conducive to determinations under difficult or doubtful 

 circumstances ; it obviates the contingency of deteriorated results arising 

 from the proximity of the experimenter, and it effects these objects with 

 less consumption of ether — an expensive article in the British Isles. 

 The essential parts, being small and compact, are fitted for the pocket 

 on expeditions of research. It does not profess the extreme sensibility 

 of Regnault's, with which its inventor can observe to the twentieth 

 part of a centigrade degree; nor can I see much use in such sensibility, 

 when used in an atmosphere which fluctuates every moment within a 

 few yards of the instrument — so much so that, when a moist breeze is 

 passing, the dew point cannot be ascertained at all : perhaps the eighths 

 of a degree (discernible on my scale) is sufficient for any attainable 

 object. 



The Psychrometer, when intended for continuous action, should be 

 so arranged that the cambric, being thoroughly wet, shall not let fall 

 more than one drop in eight or ten minutes. The rate may be regulated 

 by raising or lowering the glass fountain in the slide ring, or by taking 

 a strand or two from the woollen thread which acts as a syphon. For 

 observing the frequent variations of the hygrometric state of the atmo- 

 sphere, this self-acting Psychrometer is very convenient. In proportion 

 as the two Thermometers (one of them being wet) approach the same 

 degree of their respective scales, we learn that the atmosphere becomes 

 more moist ; and if they arrived at the same degree, the fact would 

 indicate that the atmosphere was, for that temperature, saturated with 

 moisture. But this is a rare occurrence, if it ever happen : it never has 

 occurred during my experience of several years. On one occasion, it 

 had been raining for eighteen hours, sometimes heavily ; at midnight 

 I examined the instrument, which had been placed in the open air, 

 outside a closed window : the dry bulb was 44- 75, the wet bulb 44° 5'. 

 The fraction of saturation was therefore 981 , and this was the nearest 

 approach to saturation that ever fell under my observation. The same 

 observation has been made by De Luc : he says : " The case of extreme 

 moisture existing in the open transparent air in the day, even in time 

 of rain, is extremely rare : I observed it (he says) only once, the 

 temperature being 39°." (" Phil. Trans.," 1791). 



I conceive that the mode above described has a great advantage 

 over the common method of allowing a projecting tuft of the cambric to 

 dip into a vessel of water placed underneath ; for the temperature of 

 the cambric is affected by its continuous connexion with the water 

 beneath ; but the water, in passing through the woollen thread, is cooled 

 by evaporation to the same temperature as the Thermometer itself, and 

 by the same means. 



E. I. A. PEOC. VOL. X. 3 E 



