ON SOLAE EADIATION. 39 



very early period in the experiments. The first attempt at a constant 

 source was an Argand burner with a very delicate pressure regulator, a 

 given area of the brightest part of the flame being selected as the source. 

 This proved to be a very good method of testing the variations in the 

 quality of the gas, but had to be abandoned as a constant source of radia- 

 tion. It was also objectionable on account of the difficulty of keeping 

 the glass chimney uniformly clean, and because the excessive amount of 

 heat generated disturbed the experimental conditions, and the gas fumes 

 had the effect of tarnishing the contacts of the electrical apparatus and 

 the metallic plates used as reflectors. A pair of one hundred candle- 

 power focus-lamps were then obtained from the Ediswan Company. 

 These were designed to work on a pressure of 90 volts at an efficiency 

 of about 3-6 watts per candle, and a current of 4 amperes. They were 

 spherical in form and silvered on one half, which had the effect of nearly 

 doubling the radiating power for a given current, while at the same time 

 it ensured an almost perfect constancy in the proportion of radiation 

 reflected from the rear of the source, which had proved a difficulty with 

 the Argand bui-ner. When used as constant sources of radiation the 

 lamps were worked at a pressure of only 75 volts and a current of 

 about three amperes, supplied l)y a large storage battery of forty-four 

 cells. The battery was not used for any other purpose while the experi- 

 ments were in progress, and was capable of maintaining the required 

 pressure constant to a tenth of a volt for several hours under suitable 

 conditions of charge. The pressure on the lamps was regulated and 

 recorded during the experiments by means of an automatic recording 

 potentiometer working on a scale of one inch to the volt. The readings 

 of this instrument were adjusted by means of a Clark cell, and were 

 accurate to about one part in 5,000. One of the focus-lamps was set 

 apart as a standard, and was used only for occasional comparisons. When 

 working at a voltage so far below that for which they were designed, 

 the lamps were found to remain exceedingly constant. In the course of 

 six months' work the lamp in regular use did not vary with respect to 

 the standard by more than one per cent., and its variations over short 

 periods could easily have been controlled and corrected if the accuracy 

 so far attained in the radiation measurements had made the application 

 of such a correction desirable. The area of the incandescent grid was 

 about one square inch, and the diameter of the bulb four and a half inches. 

 The lamp was set to shine through an aperture of its own diameter in a 

 double tin-plate screen, so as to include the whole of the radiation from 

 the heated glass, but to exclude as far as possible radiation from the base 

 of the lamp and heated objects in its immediate neighbourhood. This 

 precaution was particularly important in comparing the indications of the 

 tube form of radio-calorimeter with those to the bolometric sunshine 

 recpiver intended for the direct exposure of solar radiation, as the latter 

 instrument was not provided with a screen and diaphragms for excluding 

 lateral radiation, but was intended to integrate the vertical component 

 of the whole radiation from the sky as well as that from the sun. 



Determination of the Initial Rate of Heating of the Disc. — To ensure 

 uniformity of temperature of the disc, and a sufficiently slow rate of heating, 

 it was found necessary to replace the original disc by a much thicker disc 

 the size of which was chosen to be just sufficient to catch the whole of the 

 rays incident on the aperture. Before commencing an observation, the 

 reading of the galvanometer was observed with the slider at the zero o£ 



