TKANSACTIONS OF THE SECTIONS. 3d 



returns as before, the motion being probably nearly symmetrical on both sides of the 

 axis, and the time of a complete oacillatioii twenty-four hours. If this be so, the 

 numbers in Table VI. in each of my Keports in the years 1862-66, under the head 

 of means up to 1000 ft. high, both in the clear and cloudy states of the sky, must not 

 be considered as of general application, as supposed when the Reports were written, 

 but the individual residts ii'om which the means were deduced must be grouped 

 together according to the hour of observation. 



On the Formation of Dew, and its Effects. Bij Henet Hudson, M.D. 



The author contends that the condensation of atmospheric moisture tends 

 necessarily to produce an " inversion of the normal law of its temperatm'e." From 

 a full analysis of all ]\Ir. Glaisher's balloon ascents, he shows that, in every in- 

 stance, there is an increase of temperatm-e "above the clouds," /. e. where vapour 

 is condensed. The night ascents are peculiarly interesting. That on the 2nd of Octo- 

 ber 1865 distinctly proves an increase of temperatm-e as we ascend in clear calm 

 weather, "when condensation takes place;" and the ascent on the Slst of March 

 1863 is equally conclusive in proving that, " with a blue and almost cloudless 

 sky," no such inversion of the normal atmospheric temperature may occur, simply 

 because "the dew-point, just before reaching the earth, was on this occasion 

 12i degrees below the atmospheric temperature, and was even much further 

 removed at the higher elevations." Hence there was no condensation of vapom* 

 dming this ascent, and consequently the law of " diminution of temperatiu-e with 

 increase of elevation" was never subverted. 



The author asserts that Dr. AVells, from not attaching its due weight to this 

 source of atmospheric heat, was led to adopt an erroneous theory, all his ingenious 

 experiments clearly showing that the phenomenon which required explanation in 

 " dewy nights " is "increase of temperature in ascending from the earth," and not 

 merely a chilled surface in reference to the air generally. 



If we assume terrestrial radiation as a cause, it is evident that the maximum 

 absoi-ption of heat must occur in the loicest sti-atum of the atmosphere, and this 

 could not therefore in any way contribute to " greater heat " at higher elevations. 

 Many of Dr. Wells's assumptions are plainly inadmissible ; for instance, " that 

 dense clouds near the earth must possess the same heat as the lower atmosphere ;" 

 and again, "that terrestrial radiation nmst have a ready transit through an atmo- 

 sphere which transmits solar heat copiously." Also (as if to explain the inversion of 

 the normal atmospheric temperature) he assumes " that the superficial stratimi of 

 air proceeds to radiate back to the earth its excess of heat" (acquu-ed by radiation 

 FROM the earth), " and that it is thereby more chilled than the superior strata," 

 apparently forgetting that if it radiated to the earth it must do so also to the 

 upper strata, which (in lilre manner) must be considered to radiate both upwards 

 and downwards, and his hj'pothesis is therefore utterly worthless for explaining 

 the phenomena. The chilling effect of air descending from the higher regions 

 appears to be altogether ignored by Dr. WeUs, and he assumes that clouds act as 

 radiators of heat to the earth, instead of merely " screening it " from the extreme 

 chilling etherial influences (in a manner similar to the "raised board " m his own 

 experiments), and yet he admits that " during nights, generally clear, if the zenith 

 be occupied by a cloud for only afeio minutes, a thermometer on the grass will rise 

 several degrees." A cloud " in the zenith " may well act as a screen and produce 

 such a result; but its influence cannot be attributed to "radiation of heat to the 

 thermometer." It merely interferes to diminish the chUl from above, and thus 

 enables the heat of the earth to regain its ascendency in warming the surface. The 

 author also adverts to the fact observed in Mr. Glaisher's last recorded ascent 

 (29th May 1866), that " at the height of 6200 feet (the sun having set nearly 

 twenty minutes) the temperature was about 6 degrees warmer than at the same 

 elevation an bom- before." 



In page 6 (Casella's edition), Dr. Wells -writes, " Dew probably bey ins to appear 

 upon grass, in places shaded from the sun, during clear and calm weather, soon 

 after the heat of the atmosphere has declined ; " and again, " I have frequently 

 felt yrass moist, in dry weather, several hours before sunset-" on the other hand, "I 



