1866. | Adams’ Recent Astronomical Discovery. 511 
posed in a day, to secure gelatine reliefs. This is the first practical 
application of the electric light to the commercial working of 
photography, its constancy rendering it here more valuable than an 
uncertain sunlight. 
The purely scientific interest of this discovery has been scarcely 
touched upon. Of this more will be said hereafter. To physicists 
and experimentalists in magnetism the gigantic magnetic forco 
developed in the three-ton magnet will afford an opportunity, which 
will doubtless at once be seized, of repeating and extending the 
classical researches of Dr. Faraday in Diamagnetism and the 
Magnetic Condition of all Matter. 
IV. ADAMS’ RECENT ASTRONOMICAL DISCOVERY. 
By Ricuarp A. Procror, B.A., F.R.A.S. 
IF our earth, like the three other planets revolving within the zone 
of asteroids, had been without a satellite, mankind would in all pro- 
bability have remained for ever in ignorance of the great yet simple 
law of universal gravitation. Now, indeed, that the discovery of 
the law has led to such vast progress in observational astronomy, 
and to so great an extension of the methods of infinitesimal analysis 
which alone enable mathematicians to deal successfully with the 
profound problems of physical astronomy, we know that the motions 
of the planets afford available and sufficient means of testing and 
establishing the theory of gravitation. But even to Newton's 
gigantic intellect the task of founding the theory upon such evi- 
dence alone would have been too vast. Conscious of this, we find 
him again and again impressing upon Flamstead the necessity of 
supplying him with lunar observations sufficient im number and 
accuracy to afford the necessary evidence in favour of the theory he 
wished to establish. 
And as the moon afforded such important assistance when the 
theory of gravitation was first founded, so a careful comparison of 
the lunar movements with those which result from theoretical con- 
siderations has led to many valuable discoveries. Our determinations 
of the most important astronomical elements, wiz. the magnitude, 
mass, and figure of the earth, and its distance from the sun and 
moon—‘ elements,” says Laplace, “the knowledge of which had 
been the fruit of long and troublesome voyages in both hemispheres”— 
have received most important corrections or confirmations from 
exact methods of astronomical investigation applied to the lunar 
motions. It seems not unlikely that by the same means a yet 
more important element, the length of the sidereal day, which has 
