375 
second, when an eclipse will occur, if it should be thousands 
of years hence, it is taken for granted that all they teach must 
be true ! But while a child may look to the dial of a time- 
piece, and tell us to a second when the pointer will cover a 
certain mark, not one among ten thousand of grown men 
can go behind the dial, and explain how the causes operate by 
which the hands or pointers are moved. So may a very poor 
thinker calculate the time of a transit, or an eclipse, while the 
loftiest intellect becomes bewildered, and is lost in trying to 
prove even the existence of those forces on the reality of 
which the fundamental doctrines of physical astronomy 
depend. The noblest minds are overtaxed when honestly 
attempting to tell us whether there is such a thing as centri- 
fugal force, and what it really is, which is called te gravitation.^ 
No one has gone behind the scenes, and seen how the highest 
authorities in astronomy are situated, without seeing that the 
physics of this science are as unsettled and uncertain as those 
of geology itself. But we gladly look into its teachings 
notwithstanding. 
Mr. Croll, of the Glasgow Andersonian University, has 
presented the world of science with the best phase of one of 
the most interesting of all theories from this quarter.* Sir 
Charles Lyell has given Mr. Croll great credit for his 
labours in this matter, as one who has pointed out a real cause 
hitherto neglected in the calculations of geologists ; and 
although we cannot accept the conclusions at which he arrives, 
we must acknowledge our admiration of this writer. His idea, 
in essence, may be briefly stated. Our globe in being carried 
round the sun, as modern astronomy teaches, has a path which 
is not a circle, but an ellipse. This, of itself, causes the earth 
to be nearer the sun in certain parts of its orbit, and farther 
away in others. But this elliptical path of the earth does not 
always maintain the same relation to the sun as a centre ; it 
changes continually, and in the course of time, the aggregate 
of change is very considerable. At one time, the earth, at its 
nearest approach to the sun, is vastly nearer, and, at its 
farthest departure, vastly farther from that source of heat than 
it is at other times. The difference, as it is calculated by 
astronomers, is expressed in millions of miles. This element 
alone, however, would not give us any reason which could 
account for a change of temperature on the surface of the 
globe, because the motion of the earth being quickened in pro- 
portion to the nearness of its path to the sun, the amount of 
heat which it receives is the same when it is nearest as when 
* See the Bender for October 14th and December 2nd and 9th, 1864 ; also 
Philosophical Magazine, 1866, pp. 26, 27, 28, and 30. 
