1835.] 



Meteorological Ohservatiom, 



'47 



that many important meteorological phenomena are owing. 

 It is the cause of hoare-frost^ and of dew, and even within 

 the torrid zone, it sometimes causes a reduction of tem- 

 perature sufficient for the formation of ice. — It is thus that 

 ice is produced in Bengal, and not by evaporation as fre- 

 quently supposed. 



To measure this radiation, Sir John Leslie invented an 

 instrument, to which he gave the name of (Ethrioscope. 

 (Fig.3.) It consists of a differential thermometer having one 

 of its balls in the lower focus of a polished metallic cup of 

 an oblong spheroidal shape ; while the section of a horizon- 

 tal plane passes through the upper focus, and defines the 

 orifice. — Having found this instrument exceedingly liable 

 to derangement upon being moved about, I have contrived 

 the following modification of it, which possesses the great 

 advantage of being perfectly portable, and is equally accu- 

 rate. — The metallic cup* of Professor Leslie, is adapted to a 

 delicate thermometer of the annexed form,(Fig. 4.) with the 

 millesimal scale, and filled with colourless spirits of wine, so 

 as to render it insensible to the rays of light. When this 

 instrument is exposed to the open air, it will at all times 

 Indicate more or less diminution of temperature from radia- 

 tion, which will be measured in millesimal degrees, by the 

 difference between it and another thermometer shaded 

 from the sky. It may be observed along with the ther- 

 mometer at 10 A. M. and 10 p. m; but, at the former 

 hour, it will be necessary to shade it completely from the 

 rays of the sun. The greatest degree of radiation, or the 

 lowest depression of temperature caused by it, may be 

 also easily ascertained, by having the CEthrioscope furnished 

 with a small glass index, like that of the minimum regis- 

 tering thermometer, to indicate the greatest cold. Those 

 who cannot procure the instrument I have just described, 

 may substitute for it a common thermometer with a naked 

 ball, placed on some cotton or wool on the ground, and 

 freely exposed to the sky. When compared with a shaded 

 thermometer it will indicate the diminution of temperature 

 due to radiation, and if a minimum registering thermometer 

 be employed in the same way, it will give the greatest de- 



* This is generally made of brass ; but silver is preferable, from being less lia- 

 ble to tarnish. 



