278 THE POPULAR SCIENCE MONTHLY. 



The radiation of a candle 6 inches off, therefore, weighs or presses 

 the two square inches of blackened pith with a weight of 0.001624 

 graiu. In iny own laboratory, working with this torsion balance, I 

 found that a candle 6 inches off gave a pressure of 0.001772 grain. 

 The difference is only 0.000148 grain, and is fairly within the allow- 

 able limits of a lecture experiment. But this balance is capable of 

 weighing to far greater accuracy than that. You have seen that a 

 torsion of 10,021 balanced the hundredth of a grain. If I give the 

 fibre 1 more twist the weight is overbalanced, as shown by the move- 

 ment of the index-ray on the ceiling. Now 1 of torsion is about the 

 l0 ^ 0Q part of the whole torsion required by the y^j- grain. It repre- 

 sents, therefore, the To ^ o0 part of the yJ-g-, or the millionth part of a 

 grain. 



Divide a grain-weight into a million parts, place one of them on 

 the pan of the balance, and the beam will be instantly depressed ! 



Weighed in this balance the mechanical force of a candle 12 inches 

 off was found to be 0.000444 grain; of a candle 6 inches off, 0.001772 

 grain. At half the distance the weight of radiation should be four 

 times, or 0.001776 grain; the difference between theory and experi- 

 ment being only four-millionths of a grain is a sufficient proof that 

 the indications of this instrument, like those of the apparatus previ- 

 ously described, follow the law of inverse squares. An examination 

 of the differences between the separate observations and the mean 

 shows that my estimate of the sensitiveness of this balance is not ex- 

 cessive, and that in practice it will safely indicate the millionth of a 

 grain. 



I have only had one opportunity of getting an observation of the 

 weight of sunlight: it was taken on December 13th, but the sun was 

 so obscured by thin clouds and haze that it was only equal to 10.2 

 candles 6 inches off. Calculating from this datum, it is seen that the 

 pressure of sunshine is 2.3 tons per square mile. 



But, however fair an equivalent ten candles may be for a London 

 sun in December, a midsummer sun in a cloudless sky has a very dif- 

 ferent value. Authorities differ as to its exact equivalent, but I under- 

 estimate it at 1,000 candles 12 inches off. 



Let us see what pressure this wall give: A candle 12 inches off, 

 acting on 2 square inches of surface, was found equal to 0.000444 

 grain; the sun, equaling 1,000 candles, therefore gives a pressure of 

 0.444000 grain ; that is equal to about 32 grains per square foot, to 2 

 cwts. per acre, 57 tons per square mile, or nearly 3,000,000,000 tons 

 on the exposed surface of the globe sufficient to knock the earth out 

 of its orbit if it came upon it suddenly. 



It may be said that a force like this must alter our ordinary ideas 

 of gravitation ; but it must be remembered that we only know the 

 force of gravity as between bodies such as they actually exist, and we 

 do not know what this force would be if the temperatures of the gravi- 



