242, REPORT—1896. 
not be advantageous to substitute for the internal thermometer a thermo- 
electric arrangement whereby the solar radiation should be measured by 
the deflection of a galvanometer. A thin disk of blackened copper was 
fixed in the position previously occupied by the flattened bulb of the 
internal thermometer, and two wires led from this disk, namely, a 
platinoid wire from behind the middle point of the disk and a copper 
wire from the edge, the second junction of the two metals being embedded 
in the solid copper of the case, the temperature of which was given by the 
case-thermometers. A d’Arsonval galvanometer was intercalated in the 
thermo-electric circuit, and the difference of temperatures of the two 
junctions was given by the deflection of the mirror, which was read by 
eye by means of a divided scale. This arrangement was found to work 
in a very satisfactory manner ; the observations could be taken in @ 
shorter time than with the thermometer ; and on reducing the results by 
the formula given in a former report it was found that the numbers 
obtained fora magnitude theoretically proportional to the radiation came 
out very consistent with one another when they were deduced from 
different trios of readings taken on the same occasion. Professor McLeod 
had not, however, sufficient leisure to continue the experiments as he 
wished, and Mr. W. E. Wilson took charge of the instrument with a view 
to continue the experiments. 
Mr. Wilson modified the apparatus by introducing an arrangement by 
which the light reflected by the mirror of the galvanometer, instead of 
serving for eye observations, was received on a photographic plate which 
descended by clockwork, and recorded the deviations of the mirror at 
times which were recorded by a fixed light falling on the plate, which was 
interrupted at each second, so that the former light traced out a curve, 
the ordinates of which corresponded to the deflections, while the abscisse 
gave the time. 
In this manner very neat curves were obtained, which gave a perma- 
nent record of the observations. This record was of course exempt from 
possible errors of reading, and could be dealt with at leisure. In a later 
arrangement the inter ruptions at each second were recorded on the curve 
itself as well as on the line of abscissx, a method which presents certain 
advantages for the subsequent reduction. 
In order to obtain a base line corresponding to an equality of tempera- 
ture of the disk and the case, the plate was started, and the permanently 
tixed light and that reflected from the mirror not yet deflected were 
wllowed to record themselves a few seconds before the sun’s rays were 
allowed to fall on the plate. The latter gave a short straight line, paralle? 
to the axis of abscissee, corresponding to no deviation, from which the 
curve started when the light of the sun was let on. 
The curves obtained in the preliminary trials were sent to Sir George 
Stokes for reduction. The rapidity of the change from the straight line 
traced before incidence of the sun’s rays to a curve which showed no sign 
of the discontinuity of the initial conditions showed that the effect of the 
inertia of the galvanometer was practically insensible, provided at least a 
very few seconds were allowed for the instrument to get into train, that 
is, provided a minute portion of the curve, near the point cf sudden 
change in the conditions, were excluded in the reduction. 
The galvanometer was dead-beat, but it is conceivable that the 
damping force might have been such as to cause a sensible difference 
between the angular position of the mirror of the galvanometer and that 
