at the total solar Eclipse in 1882 373 



'No allowance has been made for loss. If we increase the 

 working value of the sun's rays from 0-87 to ro horse-power, 

 and take the earth's mean distance from the sun's centre to 

 be 212 times the radius of the sun, the radiation emitted by 

 one square metre of the sun's surface is spread over, in round 

 numbers, 45,000 square metres of the earth's surface. There- 

 fore the intensity of the radiation of the sun's surface is 

 equivalent to at least 45,000 horse-power per square metre. 

 This number, especially when used in connection with so very 

 small a surface as one square metre, conveys no definite idea 

 to the mind. The following consideration may assist in 

 giving definition to our conception. The specific gravity of 

 solid iron at ordinary terrestrial temperatures is about 7-5 ; 

 therefore one cubic metre of it weighs at the earth's surface 

 7500 kilogrammes. Taking the force of solar gravity at the 

 sun's surface to be twenty-eight times that of terrestrial gravity 

 at the earth's surface, one cubic metre of cold solid iron on the 

 sun's surface would exercise a pressure of 210,000 kilogrammes. 

 To lift this mass through one kilometre against solar gravity 

 would involve the expenditure of 2iox io 6 kgm. of work: 

 and if this amount of work were done in one minute, the 

 engine employed would have to develop 46,667 horse-power. 



Further, the heat which is equivalent to 2iox io 6 kgm. 

 of work is 494,100 kilogramme-degrees (kg. C). When iron 

 is burned in oxygen so as to form the magnetic oxide, the 

 heat evolved is given by the thermochemical equation 



Fe 3 + 4 = Fe 3 4 + 2647 kg. C. 



Using this constant, we find that the mass of iron which by 

 its combustion would furnish the above amount of heat, would 

 weigh on the surface of the earth 313-5 kilogrammes, and 

 would occupy a volume of 0x341 8 cubic metre, or I square 

 metre x 4- 1 8 centimetres. Therefore the heat required could 

 be produced by burning 4*18 centimetres of liquid iron on 

 a hearth of I square metre per minute. With a supply 

 of oxygen at high pressure this would not seem to be an 



1 Extract from an account of this paper given in Nature (1901), vol. Ixiii. 

 p. 548- 



