488 Mr. Charles Tomlinson's Remarks 



mining the dew-point, which has generally been assigned to 

 Dalton under the date 1801. 



But probably among all the researches into the phenomena 

 of dew those of M. Melloni are best adapted to furnish an 

 answer to Mr. Aitken. His observations were made in the 

 autumn of 1846 in the valley of La Lava, situate between 

 Naples and Salerno. It is curious to note that the Austrian 

 and Bourbon Governments, in their dread of novelty, would 

 not allow a new text-book on Physics to be introduced into 

 colleges and schools ; so that the old theory was taught, 

 namely, that dew rose from the earth. Melloni, in order to 

 show that the laws of radiation are the same in Italy as in 

 other countries, where there is more political liberty, under- 

 took these researches *. 



The early experimentalists seemed to pride themselves 

 on the great differences between the thermometers on the 

 grass and other substances, and the thermometer suspended 

 in the air above them, as when Wells assumes a kind of 

 injured tone in noticing that Six had obtained a greater dif- 

 ference than he had done, viz. 16° F. f; whereas Melloni rather 

 prides himself on the smallness of this difference, and main- 

 tains that the great depression of temperature, as observed by 

 Wells and others, arose quite as much from the radiation of 

 the glass and other materials of the thermometers, as from 

 that of the substance under examination \. Nor does he find 

 it necessary, in the economy of nature, that vegetation should 

 cool down so much below the temperature of the air as had 

 hitherto been supposed ; since a difference of one or two de- 

 grees 0. would, in most cases, suffice to condense the moisture 

 of the air upon grass and the tender surfaces of leaves. He 

 answers the theory which supposes that, as the cold varies 

 from 1° to 10° C. with the amount of shelter or exposure, so 

 certain plants are bedewed while others are quite dry, by 

 denying the fact ; for we have either the entire absence of 



* " On the Nocturnal Cooling of Bodies exposed to a Free Atmosphere 

 in Calm and Serene Weather, and on the resulting Phenomena near the 

 Earth's Surface," read to the Royal Academy of Naples on the 23rd 

 February, and 9th and 16th March, 1847. A French translation of 

 these Memoirs was made under the Author's superintendence for the 

 Annates de Chimie et de Physique for February and April 1848. An 

 excellent English translation appeared in Taylor's ' Scientific Memoirs,' 

 vol. v. 1852. 



t Mr. Aitken also notices a difference on dewy nights of from 10° to 

 18° F. between two thermometers, one placed on the grass and the other 

 under the surface among the stems, but on the top of the soil. 



X Melloni took extraordinary pains to prevent his thermometers from 

 sharing in the radiation of the bodies, the amount of whose cooling they 

 had to measure. 



