588 
THE EAEL OF EOSSE ON THE EADIATION 
and their back solderings were protected from sudden changes of temperature by being 
imbedded in wax. More care has been taken to protect the apparatus from the wind ; 
and with that view, during the last two years, the cloth which had been hitherto used 
to cover the lattice-tube was prolonged to about 5 feet beyond the end of the tube, or 
6 feet outside the piles. The three-foot telescope was employed in these as in the former 
experiments. 
The observations were made by directing the telescope towards the moon, so that by 
a slight motion the faces of the piles were alternately exposed for a period of one minute 
to the radiation from the part of the sky containing the moon’s disk, while the other 
pile was exposed to that from an adjacent circle of sky. The galvanometer was read off 
just before each motion of the telescope. Thus the difference between two successive 
readings of the galvanometer exhibited the heat-effect on double the scale that would 
have been possible with one pile. After three or four preliminary settings of the tele- 
scope had been made, to give the assurance that every thing was in working order and 
the needle of the galvanometer vibrating in equal arcs on each side of the zero, eleven 
readings of the galvanometer, giving ten differences, were taken in succession. The 
sidereal time was noted and the altitude read off on a quadrant attached to the telescope, 
graduated to half degrees, and allowing the estimation of tenths. As far as was at all 
compatible with the progress of the observations all cloud was avoided, and no readings 
were taken unless the sky near the moon was clear or very nearly so. The zenith-distance 
for the middle of each set of ten differences was obtained by a graphical process from the 
altitudes actually observed, and the arithmetical mean (G) of the differences was taken 
as the corresponding heat-effect. 
These quantities, and others of which an explanation is given further on, are entered 
in the following Table. 
Journal of Observations and preliminary Reductions. 
We have : — 
In column III. the observed deviation of the needle. 
In column IV. the moon’s distance from the point directly opposite to the sun. 
In column VII. the logarithm of the factor for reducing the galvanometer-readings 
from tangent to arc , it being assumed that the latter w 7 as proportional to the heating- 
effect. 
In columns VIII. & IX. the respective logarithms for reducing each set of readings for 
change of phase and of the moon’s apparent semidiameter to what it would have been 
had the observation been made at the time and under the circumstances given in columns 
XI., XII., & XIII. ; so that we obtained the heating-effects (given in column X.)of the 
moon at various zenith-distances on each night, independent of all other causes of 
variation. 
The remaining columns require no explanation, beyond what will be found in the pages 
which follow the Table. 
