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IX.—ON THE CHANGES OF THE RADIATION OF HEAT FROM THE MOON 
DURING THE TOTAL ECLIPSE OF 1884, OCTOBER 4, AS MEASURED 
AT THE OBSERVATORY, BIRR CASTLE. By OTTO BQ{DDICKER, Ph.D. 
(Piares IX. and X.). [Communicated with a NOTE by the EARL OF ROSSE. ] 
[ Read, April 20, 1885. ] 
1. Tue total eclipse of the moon on the 4th of October, 1884, afforded a welcome 
opportunity of testing the changes which the radiated lunar heat undergoes with 
the rapid change of the illuminated portion of the moon’s surface. The interest 
attached to these observations was so much the greater, as no lunar eclipse had 
hitherto been sufficiently favoured by the weather as to admit of any decisive 
results. It was exceedingly fortunate, therefore, that thirty-two minutes before 
the beginning of the total phase the sky, till then thickly covered with Cirro- 
eumuli, cleared suddenly, and remained most perfectly and exceptionally trans- 
parent throughout the night. And, as also a perfect calmness prevailed all night, 
it may be said that the following eclipse-observations were made under the best 
attainable atmospheric conditions. 
2. The apparatus used was the same as described by the Earl of Rosse in the 
Paper ‘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,” published in 
the Philosophical Transactions of the Royal Society in the year 1873. The 
galvanometer-needle had, however, been taken out and repaired since that time, 
so that the figures given below are not directly comparable with those published 
in 1873. Another circumstance, which would also. prevent this comparability, 
was discovered some time before the date of the eclipse. I had, namely, observed 
that the galvanometer indicated much slighter heat effects than it used to (ceteris 
paribus) during my observations of 1881, 1882, and 1883. As no reason for this 
could be found in spite of most careful and repeated examinations, and as the 
readings, iter se, were quite consistent, I raised the directing magnet from its 
usual distance of three inches to that of four inches, thus increasing the sensi- 
tiveness of the galvanometer to 2°24 as compared with unit in the former case.* 
* This proportion is based upon a very extensive series of hot tin observations which I carried on 
in 1881, and have at present under discussion. 
TRANS. ROY. DUB. SOC., N.S. VOL. III. 2U 
