3G4 
MESSRS. W. E, WILSOR" AND P. L. GRAY 
the temperature of the platinum is raised until a balance is obtained, indicated by 
the index spot of light returning’ to its zero on the scale of the radio-micrometer. 
At this same moment the temperature-scale of the meldometer is read, the local 
time of the observation is noted (to obtain the altitude of the sun), and a reading on 
the heliostat is made, by which the angle of incidence of the sunlight on the mirror 
can be calculated. 
Fig. 1. 
An exactly similar process is then gone through with the sun shining in the upper 
circuit and the platinum in the lower, and the results of each observation are 
separately calculated. 
Then if = the radiation in our arbitrary units, corresponding to a balancing 
temperature, 
A = the ratio of the total heat to the amount transmitted at the observed altitude 
of the sun, 
1) = the ratio of the incident radiation to that reflected from the mirror of the 
heliostat, 
