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XVI. The Bakerian Lecture. — On the Radiation of Heat from the Moon, the Law of 
its Absorption by our Atmosphere, and of its Variation in Amount with her Phases. 
By the Earl of Rosse, F.R.S. 
Received February 3, — Read March 27, 1873. 
In the years 1869 and 1870 I communicated to the Royal Society the results of a series 
of experiments made with the view of determining, if possible, the amount of radiant 
heat coming to the earth from the moon in various conditions of phase, and the nature 
of that heat as regards the average refrangibility of the rays. Though more successful 
than I had at first been led to expect, the imperfect accordance between many of the 
observations still left much to be desired, and the novelty and importance of the subject 
appeared sufficient to render it advisable to pursue the investigation with greater care 
and closer attention to details than had hitherto been deemed necessary. 
Since the conclusion of the series of observations which form the subject of the second 
paper above referred to, nothing (with the exception of a short series of observations in 
August and October 1870, of which mention is made towards the end of this paper) was 
done towards pursuing the subject till the spring of the following year (1871), when the 
series of observations which form the subject of the present paper were commenced, the 
same apparatus (only slightly modified) being used and the same method of observation 
adopted ; but, with the view of obtaining an approximate value of the absorption of the 
moon’s heat in its passage through our atmosphere, and of rendering possible the satis- 
factory comparison of observations made at different zenith-distances of the moon, the 
observations were in many cases carried on at intervals at all possible zenith-distances 
on the same night, and the most favourable opportunities for observing the moon at very 
different zenith-distances in various conditions of the atmosphere were not lost. 
Before proceeding further it may be well to mention the small changes in the appa- 
ratus already referred to. 
The piles instead of being, as before, placed in two separate circuits or loops, united 
close to the terminals of the galvanometer, have, since the beginning of August 1870, 
been placed in one and the same circuit, the position of the poles of the second pile 
relative to the galvanometer-circuit being, as before, the reverse of the first ; and further, 
to protect the piles more effectually from draughts of air, they and the small concave 
mirrors were enclosed in a box with glass'* sides, that side next the large speculum being 
still (unless otherwise specified) left open. The piles were single pairs of small cross 
section of the kind described in the ‘Proceedings of the Royal Society,’ No. 122, 1870, 
* Of glass to enable the assistant to see when the moon’s image was central on each concave mirror. 
MDCCCLXXIII. 4 K 
