450 MELLONI ON THE NOCTURXAL COOLING OF BODIES. 



when looked at in a dark place by means of a light placed behind 

 the strip of ivory, a circumstance of great importance in nocturnal 

 observations. But a quality still more precious in these ther- 

 mometers constructed with corks, consists in the great facility 

 which they afford for comparison between the temperatures of 

 the air and of bodies which radiate towards the celestial space. 

 For this purpose, a small vase of silver or brass is taken, similar 

 to a common sewing-thimble, whose surface is to be smooth and 

 polished, and its dimensions sufficient to receive the bulb of a 

 thermometer, and then fit on witli friction to the lower extremity 

 of the small cork cylinder. The thermometer having thus its 

 reservoir protected by a metal armature, and the tube by an en- 

 velope of the same nature, loses almost completely its emissive 

 power, as we shall soon see, and consequently furnishes the true 

 temperature of the stratum of air in which it is plunged. Also, 

 if we cover the exterior surface of the armature with lamp-black 

 or a varnish, the emissive power of the apparatus is raised to its 

 maximum ; and the thermometer, being properly placed in free 

 air, descends below the surrounding temperature by radiation 

 towards the upper regions of the atmosphere. All this is clearly 

 manifested by the following experiments. 



On the 17th of last September (1846) the weather was fine 

 and calm in the valley named La Z/«v«,, situated between the 

 cities of Naples and Salerno. At 9 o'clock in the evening I ex- 

 jjosed on a terrace raised 15 metres above the ground, three 

 thermometers sensibly equal, armed in the manner I have just 

 mentioned ; two had their armature polished, that of the third 

 was coated with lamp-black. These thermometers wei'e placed 

 horizontally, and each of them had its reservoir placed at the 

 bottom of a vessel formed of tin-plate, and of the shape of a trun- 

 cated cone inverted, the radius of its lower end being 2 cen- 

 timetres, and that of the upper end 7 centimetres. These 

 vessels, which were 8 centimetres high, were supported by 

 tripods 50 centimetres in height, formed of slender tin-plate 

 tubes, which, in addition to their firmness, possess the advantage 

 of having but little matter in their transverse section ; and hence, 

 by affording very little communication of heat with the ground 

 beneath them, almost completely isolate the bodies they support. 



In order to introduce the thermometer horizontally into the 

 recipients and keep them in this ])osition, each vessel had a lateral 

 opening made close to the I)ottom, and furnislied on the inside 



