40 REPORT — 1869. 



have scarcely ever kno^vn dew iipon grass to exhibit visible drops before the sun 

 was very near tlie horizon, or to be very copious till some time after sunset." 

 Again (page 27) he wi-ites, " According to a few observations made by me, the 

 greater coldness of gTass than that of the air begins to appear, in clear and calm 

 weather, in places sheltered from the sun, soon after the heat of the atmosphere 

 has declined. Hence it is evident that the " dew-process'' was in active operation 

 long before he could possibly find any addition to the weight of his wool, and, as 

 regards the amount of heat received by the air, it is sufficient to point out that the 

 latent heat of 20 gi-ains of moisture (which he frequently found in 10 grains of 

 wool) woidd raise the temperature of 20 cubic feet of air about 8 degrees ; hence 

 we see what an enormous amount of heat must have been given out by the vapour 

 deposited on the grass-plot, and wonder how such a "vera causa" has been ignored 

 in the explanation of the phenomena ; and yet Dr. Wells only notices the fact in 

 the following terms (page 53) : — " The formation of dew, indeed, not only does not 

 produce cold, but, lilie every other precipitation of water from the atmosphere, 

 produces heat." He knew that heat was eliminated, and that it was not commu- 

 nicated to the surface, and yet failed to see in it the source of atmospheric increase 

 of temperature. In page 72 Dr. Wells admits that " bodies exposed in a clear 

 night to the sky must radiate as much heat to it during the prevalence of wind 

 as they would do if the air were altogether still, but in the former little or no cold 

 will be observed upon them above that of the atmospheric." That is to say, there 

 was little dew, and therefore the air had received little or no heat from this som'ce. 

 If Dr. Wells had given actual temperatures, instead of merely " differences," we 

 should have had far more satisfactory data to go upon. 



In conclusion the author insists that Provost's Theory of Exchanges cannot be 

 reconciled to the different " radiation energies " of surfaces. Imagine a thermo- 

 meter in the focus of a metallic mirror, and a cubic canister (two adjacent sides 

 being bright metal and the other two varnished) placed angvlarhj in the Ime of 

 the axis of the mirror ; it will be admitted (supposing the temperatures of mirror, 

 thermometer, canister, and air to be identical) that, whether the metallic or var- 

 nished sides are " radiating " to the mirror, there will be no efiect on the focal 

 hall, and yet the " radiating energy " of the varnished sides is manifold greater 

 than that of the metal ; why, then (on Prevost's theory), does not the focal ther- 

 mometer indicate this increased effect, as concentrated by the mirror ? It will be 

 understood that the angular position of the canister is to meet any hypothesis of 

 the " metal reflecting the thermometer's radiation hack again to it." A fortiori 

 (under the above arrangement), if the canister alo7ic be reduced below the tempe- 

 rature of the air, why does the varnished surface chill the thermometer more than 

 the metal, although the former is so much more potent in radiating heat ? 



^\.bove thii'ty-four j'ears ago the author devised what he considered an cxperi- 

 mentum crncis on this subject. He had a vessel made of zinc, with one side of it 

 a mirror. This was filled with water at 173° F., and one ball of a delicate differ- 

 ential thermometer haviug been placed in the focus of the miiTor, the other ball 

 was moved round until tlie instrument marked zero. A large tin screen was in 

 front of the mirror, at a distance of about 6 feet. The temperature of the room 

 being 55°, a cubic canister (as described) containing water at 67° was placed just 

 in front of the screen ; the focal ball showed increase of temperature, and the rise 

 was greatest with the varnished sides of canister. On moving the canister nearer 

 to the mirror, these effects diminished, and at a certain distance the effect (from 

 hoth sides) became nil. On being moved nearer still the canister began to act as 

 if it were [a cold hod;/, and the varnished surface produced the gTeater chilling 

 effect. A paper, containing these results, was' read by the author in Section A at 

 the first Dublin Meeting of the British Association (in 1835), and was honoured 

 by having been printed in extcnso, amongst the Eeports. The author did not then 

 call attention to tlie consequences of these experiments, saving that he considered 

 them only explicable on a " Wave Theory." 



As Prevost's theory is still received by the first physicists of Europe, the author 

 no longer hesitates to assert that it is not true. The facts adduced appear to 

 him to demonstrate that when any body is of the same tempcrattrre as the medium, 

 there is no "radiation," but when it is either warmer or colder than the medium, 



