44 KEPORT — 1900. 



water-jacket, the conditions of cooling are disturbed l)y the excessive 

 intensity of the radiation, and the final excess of temperature is much too 

 large to permit of the application of the elementary theory of the method. 

 For these and other reasons it appeared preferable to employ the automatic 

 recording instruments already described ' for direct exposure to sunshine, 

 and to calibrate the receivers in absolute measure by exposure to the 

 radiation of the focus lamps, which could be satisfactorily determined by 

 the absolute method. 



Bolometric Sunshine Receivers. — These instruments are intended for 

 recording on an arbitrary scale the vertical component of the radiation 

 from the whole sky as well as the sun. This vertical component measures 

 the heat received by the soil, and is probably the factor which chiefly 

 influences the meteorological conditions at any part of the earth's surface, 

 so far as they depend on radiation. It is comparatively useless for this 

 purpose to record merely the normal intensity of solar radiation, as the 

 heat actually received by the earth's surface depends so greatly on the 

 altitude of the sun and the state of the sky. It is proved by actual 

 experiment with these receivers, although it is by no means obvious 

 a priori, and will perhaps scarcely be credited at the first statement, that 

 the heat received by reflection from the sky under certain conditions may 

 amount to more than 40 per cent, of the whole vertical component. This 

 lieing the case, the readings of an instrument which records only the 

 normal intensity of direct sunshine, excluding the radiation from the sky, 

 might give a very incorrect account of the total quantity of heat received 

 by the soil. The form of bolometric receiver adapted for recording the 

 vertical component consists of a difierential pair of flat platinum ther- 

 mometers, one blackened and the other bright, placed side by side in the 

 same horizontal plane. The difference of temperature between the two, 

 which is automatically recorded, is approximately a measure of the 

 intensity of the vertical component of the radiation to which they are 

 exposed. It would of course be possible, by providing the instrument 

 with a water-jacketed tube and an equatoi'ial mounting, to make it record 

 the normal intensity of direct sunshine, excluding the greater part of the 

 radiation from the sky ; but this would complicate the apparatus consider- 

 ably, and it is doubtful whether the record would have so direct a bearing 

 on meteorology. It is also certain that the coefficient of cooling by con- 

 vection would vary at different angles of inclination, whereas it appears to 

 be very constant in the horizontal position. 



Two of the bolometric receivers above described have already been 

 compared with the radio- calorimeter by means of the focus-lamps. They 

 were of slightly different patterns, and wound with wire of diflferent sizes, 

 six mils and four mils respectively, but they showed nearly the same 

 difference of temperature when exposed to the same radiation at the same 

 distance. This seems to show that the indications of such instruments 

 are fairly comparable, even if they are not precisely alike. As we have 

 already seen, the absorptive powers of different kinds of black do not 

 appear to differ very much for this kind of radiation. The proportionality 

 of the difference of temperature to the intensity of radiation was also 

 tested by varying the distance from the lamp, and assuming that the 

 radiation followed the law of the inverse square. This is very approxi- 

 mately true for the focus-lamps, owing to the flatness of the radiating 



' B.A. Itepori, 1898. 



