Q2 THE HAMILTON ASSOCIATION. 
mixed up with astrological fancies. But Kepler, even in error, erred 
like a man of genius. When his feet took the wrong path, his face 
often turned toward the right. He believed the sun to have a soul, 
which was constantly rotating. He also thought that between sun 
and planets there is a friendly side, and a side that is hostile; and 
that when the friendly side was turned the planets moved toward the 
sun, and when the hostile side was turned they moved from him. 
All this was fanciful enough, but here error pointed in the direc- 
tion of truth, for twenty years later Galileo saw through his telescope 
that the sun’s rotation was a reality. Newton’s theory has withstood 
more than two centuries of criticism, and is confirmed by the most 
careful observations. Eight thousand telescopic observations taken 
of the moon during a period of eighty years were compared, under 
direction of Prof. Airy, with the place at which, by Newton’s theory, 
the moon should be at the time of each observation. Each theoret- 
ical place was computed separately and independently. The work 
took a body of calculators eight years, at a cost of £4,300, and by it 
the truth of Newton’s theory was fully sustained. 
If the moon revolved around the earth, controlled solely by force 
of their mutual attraction, the calculation of her orbital motion 
would present no special difficulty to the expert astronomer. What 
would be the moon’s position in the heavens at a given future time 
could be predicted with like exactness to that of Jupiter, which has 
been given ten years in advance, to within half a second of actual 
observation. But in addition to attraction of the earth, the moon is 
influenced by that of the sun, and to a less extent by that of the 
nearest planets. Moreover, from the moon’s elliptic orbit and ircli- 
nation of the plane of that orbit to the plane of the ecliptic, the sun’s 
attraction is a force constantly varying, both in degree and direction. 
Hence calculation of the lunar motion is one of the most difficult 
tasks accomplished in the field of physical astronomy. In a letter 
to Flamsteed, Newton himself lets fall words bordering on doubt as 
to whether he should finish the task. These lunar inequalities, as 
they are called, Prof. Airy explained in his work on “ Gravitation.” 
His book was written for general readers ; and Lord Brougham, who 
tried his hand at similar work, deemed it the best account of the 
Newtonian philosophy ever written, or likely to be written. 
Besides theoretical interest of being able to predict exactly the 
