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known as radiation. The Sun sends heat to us by radiation, his 

 rays being carriers of his whole energy. This energy is mani- 

 fested as heat, or hght, or by its chemical effects solely according 

 to the object by which they may be intercepted. 



3. Analysis of the Solae Radiation. 



The solar rays can be analysed by placing a glass prism cut at 

 a proper angle in their path. They will emerge sifted, and bent 

 round towards the thicker part of the prism, forming a spectrum, 

 the dark or heat rays being the least bent of all, the violet the 

 most, and the other colours in their order, from red to violet. 

 The heat rays, though invisible to the human eye, can be 

 analysed by other means. Their chemical activity has been found 

 by Captain Abney, who has photographed, on properly prepared 

 plates, this invisible end of the spectrum, and, what seems more 

 paradoxical still, has by their means obtained a picture of a hot 

 kettle in the dark. Using the principle that an electric current 

 will flow more easily through a cold than through a hot wire, 

 Professor Langley has constructed an instrument called the 

 "volmeter," for mapping this infra-red portion of the solar 

 spectrum. Just as an exposed tooth-nerve in a cold draught of 

 air telegraphs notice to the brain, so the sensitive platinum wire 

 inserted in the circuit betrays the presence of an absorption line 

 or band in the dark part of the spectrum by deflecting the needle 

 of the attached galvanometer. By means of this instrument the 

 dark end of the solar spectrum has been traced to a distance of 

 about twenty-five times the extent of the visible rays. 



4. Measuring the Amount of Solab Heat. 



The question as to the probable amount of heat transmitted 

 from Sun to Earth now claims attention. But since heat is a 

 physical quantity, and therefore capable of measurement, 

 the expression of such measurement requires that some 

 standard or unit of heat be selected. The amount of heat 

 which will raise lib. of water, though 1° on the Fahrenheit 

 scale of temperature, is a convenient unit for our purpose. The 

 instrument first used by Sir John Herschel and Pouillet in 1838 

 to determine the " solar constant " or the number of heat-units 

 received in each unit of time by a unit of surface of the earth 

 when exposed to the Sun in the zenith is called the " pyr- 

 heliometer." By its means a known amount of matter, such as 

 water or mercury, is exposed for a known time to the action of a 

 sunbeam of known cross-section, and the rise of temperature 

 being noted, the " solar constant " can be deduced. The instru- 

 ment consists essentially of a little blackened box full of water, 

 which, by means of a suitable mounting, can be directed 

 squarely to the Sun. Inserted in the box is a thermometer, and 



