THE MECHANICAL ACTION OF LIGHT. 279 



tating masses were to undergo a change. If the sun is gradually 

 cooling, possibly its attractive force is increasing, but the rate will be 

 so slow that it will probably not be detected by our present means 

 of research. 



While showing this experiment I wish to have it distinctly under- 

 stood that I do not attach the least importance to the actual numeri- 

 cal results. I simply wish to show you the marvelous sensitiveness 

 of the apparatus with which I am accustomed to work. I may, indeed, 

 say that I know these rough estimates to be incorrect. It must be 

 remembered that our earth is not a lampblacked body inclosed in a 

 glass case, nor is its shape such as to give the maximum of surface 

 with the minimum of weight. The solar forces which perpetually pour 

 on it are not simply absorbed and degraded into radiant heat, but are 

 transformed into the various forms of motion we see around us, and 

 into the countless forms of vegetable, animal, and human activity. 

 The earth, it is true, is poised in vacuous space, but it is surrounded 

 by a cushion of air; and, knowing how strongly a little air stops the 

 movement of repulsion, it is easy to conceive that the sun's radiation 

 through this atmospheric layer may not produce any important amount 

 of repulsion. It is true the upper surface of our atmosphere must pre- 

 sent a very cold front, and this might suffer repulsion by the sun ; but 

 I have said enough to show how utterly in the dark we are as to the 

 cosmical bearings of this action of radiation, and further speculation 

 would be but waste of time. 



It may be of interest to compare these experimental results with a 

 calculation made in 1873, before any knowledge of these facts had 

 been made public. 



Prof. Clerk Maxwell, in his " Electricity and Magnetism," vol. ii., 

 p. 391, writes as follows : " The mean energy in one cubic foot of sun- 

 light is about 0.0000000882 of a foot-pound, and the mean pressure on 

 a square foot is 0.0000000882 of a pound-weight. A flat body exposed 

 to sunlight would experience this pressure on its illuminated side only, 

 and would therefore be repelled from the side on which the light 

 falls." 



Calculated out, this gives the pressure of sunlight equal to about 

 two and a half pounds per square mile. Between the two and a half 

 pounds deduced from calculation and the fifty-seven tons obtained 

 from experiment the difference is great ; but not greater than is often 

 the case between theory and experiment. 



In conclusion, I beg to call especial attention to one not unimpor- 

 tant lesson which may be gathered from this discovery. It will be at 

 once seen that the whole springs from the investigation of an anomaly. 

 Such a result is by no means singular. Anomalies may be regarded 

 as the finger-posts along the high-road of research, pointing to the 

 by-ways which lead to further discoveries. As scientific men are 

 well aware, our way of accounting for any given phenomenon is not 



