486 Mr. Charles Tomlinson's Remarks 



of the ground, such as a turf six inches square that had been 

 exposed, for some hours while dew was forming, it was always 

 found to have lost weight. 



IV. That bare soil loses nearly as much in weight as grass- 

 land. 



V. That pieces of blackened glass placed on the ground 

 remain clear of vapour, thereby showing the soil to be always 

 giving off vapour because the surface is above the dew- 

 point. 



VI. That even where radiation is strong and vapour is con- 

 densed on the soil, such vapour is supposed to proceed from 

 below, and to be trapped by the cold surface-soil. Thus the 

 under surface of clods are often covered with hoar-frost, while 

 there is little on the upper surface ; and in roads the under 

 surface of stones may be wet as well as the under sides of 

 slates placed on gravelly roads. 



VII. That the leaves of plants apparently wet with dew are 

 not really so, but the moisture is an exudation from the plants 

 themselves *. 



VIII. That the radiating powers of different bodies have 

 hitherto been erroneously stated. Black and white cloths 

 were found to radiate equally well ; soil and grass were 

 also almost exactly equal to each other ; lampblack was 

 equal to whitening ; sulphur was about two thirds that of 

 black paint ; snow in the shade on a bright day was at mid- 

 day 7° colder than the air, while a black surface at the same 

 time was only 4° colder ; but at night both radiated almost 

 equally well. 



Such is the new theory of dew, which, if accepted, must go 

 far to render nugatory the results obtained by some of the 

 most celebrated observers. It would occupy too much time 

 and space to examine the above eight propositions minutely, 

 and to compare their statements with already received results. 

 It will, perhaps, be sufficient for our purpose to give a few 

 details from those researches upon which we have hitherto 

 most relied, and leave the reader to contrast them with Mr. 

 Aitken's ingenious speculations. 



That " the ground at a short distance below the surface is 

 always hotter than the air over it/' is not a new observation. 

 In 1779 Pictet f found that a thermometer suspended 5 feet 

 from the ground marked a lower temperature on clear nights 

 than one suspended at the height of 75 feet ; he also placed a 

 thermometer with its bulb buried in the earth, and found it to 



* Muschenbroeck regarded dew as a real perspiration of plants. 

 t Lettres Physiques fyc. of M. de Luc, La Haye and Paris, 1779 ; also 

 JEssais de Physique, by M. Pictet, Geneva, 1790. 



