BoEDDiCKEK — On Lunar Radiant Heat. 507 



III. — Discussion of the Ohservations. 



1. Decrease of heat before the first contact with the penumbra. 



It might be alleged that this decrease is nothing but a result of the mode in 

 which the final heat-curve has been constructed, and is as such of a purely 

 arbitrary character, and not concordant with actual facts. A glance at Plate LIII. 

 will, however, suffice to show that such can hardly be the case. I also add that I 

 drew the curve as much as possible without any bias, keeping myself all the time 

 carefully in ignorance of the difFerent phases of the eclipse. I think it will have 

 to be admitted that the curve could hardly have been drawn differently. Even 

 the assumption that the decrease of heat did not begin until ^ 20™ or 22" 

 would not agree with the observations, and could not be made without 

 necessitating a sudden and inadmissible bend of the curve at about 4*" 23°'. 

 And further, if we assign any weight to the first observation of 2,'^ 22'7"^even 

 allowing it to be erroneous to a very considerable degree — the lunar heat would 

 be run up, as it were, to over 670 at 4'' 20°", and to about 700 at S"" 23°', or 

 about an hour before the beginning of the eclipse. Thus a still earlier and, I 

 think, altogether incredible decrease of heat would be brought about. Not that I 

 consider the beginning of the decline at 4'' 9 ■2'" as indicated by the finally 

 adopted curve on Plate LV. any more probable. For I find that at that epoch the 

 vertical distance of the Earth's centre from the nearest common tangent of Moon 

 and Sun amoimted to round 4685 miles. If we subtract from this the mean semi- 

 diameter of the Earth, it would leave 725 miles as the approximate height of 

 the Earth's atmosphere. This inadmissible amount shows that our observa- 

 tions are too much affected by disturbing influences as to admit of an accurate 

 determination of the height at which the terrestrial atmosphere begins to 

 absorb the solar heat. But they enable us to draw a lower limit for this height. 

 It is evident, namely, that at 4'' 24"", or about 3 minutes before the first 

 contact with the penumbra, the decrease of heat has definitely set in. And this 

 indicates a height of heat-absorbing atmosphere of our Earth of not less than 

 190 miles. In any case, however, this result — though of considerable interest — 

 must be received with caution until it has been confirmed by further observations. 



2. During the progress of the penumbra the decrease of heat is decidedly 

 more rapid than that of light. This must be chiefly due to the advance of the 

 Earth's atmosphere, which would absorb a greater proportion of the heat- than of 

 the light-rays. At first the heat-curve is not very steep. This was to be expected, 

 as at first only those portions of the lunar surface are cut off which have the Sun 

 near the horizon, and must, therefore, be considerably colder than the central 

 portions. As the shadow advances these central parts of the Moon become rapidly 



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