MELLONI ON THE NOCTURNAL COOLING OF BODIES. 455 



during the night below the surrounding temperature, we must 

 find out a method of neutralizing, or at least diminishing as far 

 as possible, the radiation of the thermometer which measures the 

 temperature of the air ; and it is absolutely indispensable that 

 the two instruments should be kept during the observations in 

 the same horizontal stratum of the atmosphere. 



The well-known fact of the radiating power of metals being 

 less than that of all other bodies, led several observers to cover 

 the thermometer used in measuring the atmospheric temperatures 

 with leaves of gold, silver or tin ; but these envelopes scarcely 

 satisfy the required conditions, in consequence of the extreme 

 difficulty of adapting exactly the metallic leaves on the glass, 

 without forming wrinkles or leaving some portion of the bulb 

 exposed. And then, since the thermometer enveloped in me- 

 tallic leaf was employed solely to measure the temperatures of 

 the air, and that in order to obtain the effect of radiation of dif- 

 ferent substances, they continued to use naked thermometers, 

 there ensued a new source of error in consequence of the different 

 sensibilities of these two species of thermometers, the former 

 being necessarily rather more sluggish {'paresseux) than the 

 second. Both of these inconveniences may be easily avoided by 

 preparing in the following manner all the thermometers which 

 it is intended to use in researches on nocturnal cooling. 



Procure in the first place a small cylinder of cork of fine tex- 

 ture, w hose form and dimensions are about the same as those of 

 a common cork ; let it be pierced in the direction of its axis by 

 a small hole, in which is to be introduced the extremity of a 

 thermometer graduated on its tube, and having gently pushed 

 the cork to within 5 or 6 millimetres distance from the bulb, 

 fix it firmly in this position with mastic and some small wedges 

 of wood or cardboard. The thermometer-tube is afterwards to 

 be applied to a sheet of paper to copy the scale, which is then 

 to be engraved on a very slight strip of ivory. This moveable 

 ivory scale may then be adjusted to the thermometer by means 

 of an incision made in the upper part of the cork, and securely 

 fixed by the help of two small pegs, when a perfect coincidence 

 has been obtained between its divisions and those of the glass. 

 The small strip of ivory is then fixed on the tube, as is usually 

 done on thermometers with moveable scale. In the instruments 

 thus prepared, the extremity of the column of mercury and the 

 corresponding degree on the scale are distinctly visible at a glance 



