1876.] Mechanical Action of Light. 255 
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 sunlight 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 2j lbs. per square mile. Between the 2\ lbs. de- 
duced from calculation and the 5 7 tons obtained from expe- 
riment 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 
unimportant lesson which may be gathered from this disco- 
very. 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 bye- 
ways which lead to further discoveries. As scientific men 
are well aware, our way of accounting for any given pheno- 
menon is not always perfect. Some point is perhaps taken 
for granted, some peculiar circumstance is overlooked. Or 
else our explanation agrees with the faCts not perfectly, but 
merely in an approximate manner, leaving a something still 
to be accounted for. Now these residual phenomena, these 
very anomalies, may become the guides to new and important 
revelations. 
In the course of my research anomalies have sprung 
up in every direction. I have felt like a traveller navigating 
some mighty river in an unexplored continent. I have seen 
to the right and the left other channels opening out, all 
claiming investigation, and promising rich rewards of disco- 
very for the explorer who shall trace them to their source. 
Time has not allowed me to undertake the whole of a task 
so vast and so manifold. I have felt compelled to follow 
out, as far as lay in my power, my original idea, passing over 
reluctantly the collateral questions springing up on either 
hand. To these I must now invite the attention of my 
fellow-workers in Science. There is ample room for many 
enquirers. 
