1887.] on the Sun's Heat. 3 



sun is concerned, but does not suffice to disprove the idea which was 

 so eloquently set forth by Sir John Herschel, and which prevailed till 

 thirty or forty years ago, that the sun is a solid nucleus inclosed in 

 a sheet of violently agitated flame. In reality, the matter of the 

 outer shell of the sun, from which the heat is radiated outwards, must 

 in cooling become denser, and so becoming unstable in its high 

 position must fall down, and hotter fluid from within must rush up 

 to take its place. The tremendous currents thus continually pro- 

 duced in this great mass of flaming fluid constitute the province of 

 the newly-developed science of solar physics, which, with its mar- 

 vellous instrument of research — the spectroscope — is yearly and daily 

 giving us more and more knowledge of the actual motions of the 

 different ingredients, and of the splendid and all-important resulting 

 phenomena. 



To form some idea of the amount of the heat which is being 

 continually carried up to the sun's surface and radiated out into space, 

 and of the dynamical relations between it and the solar gravitation, 

 let us first divide that prodigious number (476 X 10^^) of horse-jDower 

 by the number (6 • 1 X 10^^) of square metres * in the sun's surface, 

 and we find 78,000 horse-power as the mechanical value of the radia- 

 tion per square metre. Imagine, then, the engines of eight ironclads 

 applied, by ideal mechanism of countless shafts, pulleys, and belts, to do 

 all their available work of, say 10,000 horse-power each, in perpetuity 

 driving one small paddle in a fluid contained in a square metre 

 vat. The same heat would be given out from the square metre * 

 surface of the fluid as is given out from every square metre of the 

 sun's surface. 



But now to pass from a practically impossible combination of 

 engines, and a physically impossible paddle and fluid and containing 

 vessel, towards a more practical combination of matter for producing 



* A square metre is about 10| (more nearly 10*764) square feet, or a 

 square yard and a fifth (more nearly 1*196 square yards). The metre is a 

 little less than 40 inches (39 '37 inches = 3*281 feet = 1*094 yards). The 

 kilometre, which we shall have to use presently, being a thousand metres, is a 

 short mile as it were (*6214 of the British statute mile). Thus in round numbers 

 62 statute miles is equal to 100 kilometres, and 161 kilometres is equal to 100 

 statute miles. The awful and unnecessary toil and waste of brain power involved 

 in the use of the British system of inches, feet, yards, perches, or rods, or poles, 

 " chains," furlongs, British statute miles, nautical miles, square rod (30J square 

 yards)! rood (1210 square yards)! acre (4 roods), may be my apology, but it is 

 only a part of my reason, for not reckoning the sun's area in acres, his activity 

 in horse-power per square inch or per square foot, and his radius, and the earth's 

 distance from him in British statute miles, and for using exclusively the one- 

 denominational system introduced by the French ninety years ago, and now in 

 common use in every civilised country of the world, except England and the 

 United States of North America. The British ton is 1 *016 times the French ton, 

 or weight of a cubic metre of cold water (1016 kilogrammes). The French ton, 

 of 1000 kilogrammes, is -9842 of the British ton. Thus for many practical 

 reckonings, such as those of the present paper, the difference between the British 

 and the French ton may be neglected. 



B 2 



