MELLONI ON THE XOCTURNAL COOLING OF BODIES. 457 



with a metallic tube which covered half of the cork of the ther- 

 mometer. The stems of the thermometers and their ivory scales 

 were enclosed in cases of thin tin-plate, which fitted on to the 

 other half of the cork, passing through the exterior side of the 

 vessel, and could be taken away and replaced at pleasure, to 

 observe the indications of the instruments, and to preserve them 

 from the moisture of the atmosphere, and especially from the 

 effects of the cooling of the ivory scales and thermometer-stems*. 

 The openings in the vessels were in the first place closed by discs 

 of tin-plate. 



After being exposed for half an hour, and consequently at 9*^ 

 soothe three thermometers marked the same temperature,! 7°*Ct, 

 or to speak more accurately, they only differed from each other 

 by the same fractions of a degree (0*05 and 0"09) marked by the 

 three instruments when plunged uncovered into a large vessel 

 of lukewarm water. At 10 o'clock the thermometers were ob- 

 served again, and all three were seen to mark a temperature of 

 17°'3: at 10'^30'^the three thermometric columns indicated 17'*1. 



* If the tubes of the thermometers were left exposed to the free air, the cold 

 resulting from their radiation towards the sky might interfere with the action 

 of tlie different substances applied to the bulbs, to such a degree that it would 

 often be impossible to recognise the difference in their emissive powers. And 

 it is easy to perceive the cause of this confusion, if we reflect that in the vertical 

 position in which the thermometer is usually held, the radiation takes place from 

 all points of the surface of the tube. The cold produced on the superficial 

 layers is propagated along the sides to the bulb, and in a transverse direction 

 as far as the middle, 'i'he liquids of the thermometric column which contract, 

 descend and are replaced by a corresponding portion of particles from the bulb, 

 and a circulation is formed whose cooling effect is added to that produced by 

 the direct contact between the tube and bulb ; so that a considerable part of the 

 cold produced by the radiation of the stem is communicated to the whole mass 

 of the thermoscopic liquid and to the reservoir of the thermometer. 



•f- Wlien the experiments arc made in the midst of fields on calm and serene 

 nights, the air surrounding the thermometer is always very moist : we shall 

 hereafter perceive the cause of this great humidity. Admitting it for the pre- 

 sent as a certain fact, it evidently follows that in this case the cold transmitted 

 by the stem to the bulb of the thermometer will cause a precipitation of aque- 

 ous vapour on whatever substance covers it. Now water being endowed with 

 a great emissive power, will begin to radiate itself, and immediately to cool 

 down the thermometric reservoir, and this cold will become sensibly equal to 

 that of the most radiating bodies; so that two thermometers with uncovered 

 tubes, one of which has its reservoir coated with lamp-black, and the other gilt 

 or silvered, if exposed to the free air on calm and serene nights after having 

 presented a difference of cold in favour of the former substance, will ultimately 

 indicate the same degree of heat. 



This is the reason why some experimenters have not observed any appreciable 

 difference between the nocturnal cooling of a series of thermometers, whose 

 stems were uncovered, placed at the same height, although the reservoirs of tliese 

 tiiermometers were covered with different substances, or put into communication 

 with plates of different nature sustained by ylass cylinders. 



