254 Mechanical Action of Light. [Aprils 
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 different value. Authorities differ as to its 
exaCt equivalent, but I underestimate it at 1000 candles 
12 inches off. 
Let us see what pressure this will give A candle 
12 inches off, aCting on 2 square inches of surface, was found 
equal to 0*000444 grain ; the sun, equalling 1000 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 three thousand million 
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 ordi- 
nary 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 gravitating 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. 
Whilst showing this experiment I wish to have it distinctly 
understood that I do not attach the least importance to the 
aCtual numerical results. I simply wish to show you the mar- 
vellous sensitiveness of the apparatus with which I am accus- 
tomed 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 lamp-blacked body enclosed 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 vegetal, 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 present a very cold front, and this might suffer repul- 
sion 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 
