ON A81RONOMY. 105 



Each separate body in the system attracts every other body by the 

 same law. The earth, for instance, has a fixed mass ; it attracts the 

 moon and the sun, and each of the phmets, by a force proportional to 

 its mass, and inversely as the squares of their respective distances. 

 To illustrate this law, as it applies to the solar system, let S in 

 fig. 6 represent the sun. 



Suppose the planets placed at their relative distances, the earth's 

 distance being unity, then the figures in the upper part of the dia- 

 gram will represent the relative distances of the other planets, viz : 

 Mars 1.5, Ceres, as the mean of the asteroids, 2.8, Jupiter 5.2, 

 Saturn 9.5, Uranus 19.1, and Neptune 30. The force of the solar at- 

 traction upon them will then be represented by the figures on the 

 lower part of the diagram, which are inversely as the squares of the 

 distances. The attraction on the earth being 1, on Mars it will be yV, 

 on Ceres y|y, on Jupiter r^V, on Saturn ^Vj ^^ Uranus .. 1 o, ^^'^ on 

 Neptune g J-q. If their orbitual motions were stopped, the earth would 

 move toward the sun 900 times as fast as Neptune. 



This enables us to form a true conception of what this law is. Of 

 the nature of gravity itself we know nothing. We give the name of 

 law to the effects which it produces. Everything connectsd with the 

 history and progress of so grand a principle in nature must awaken a 

 special interest. Any law or principle which enables the astronomer 

 to determine, not with uncertainty and vague conjecture, but with the 

 utmost precision, the relative position of the heavenly bodies for ages 

 past and for ages to come, challenges our admiration. The astrono- 

 mer turns his telescope to the heavens and watches the progress of the 

 stars as they enter and cross his field of view, and marks the hour, 

 minute, and second when one of them disappears for an instant behind a 

 spider's web which is stretched across his field of view. Suppose the 

 telescope to stand unmolested, it is not difficult to predict the hour, 

 minute, and second when the same star will disappear behind the 

 same spider line after the lapse of a hundred years. The earth will, 

 in the mean time, have made a hundred revolutions around her orbit 

 of nearly 600,000,000 of miles, but the force of gravity will have kept 

 her in her appointed track. She pursues her elliptic course without 

 weariness or change. 



3. But let us glance at the history of the discovery of this great law. 

 Speculations upon the causes of the heavenly motions were probably 

 coeval with the observations which served to make them known. 

 Many of these speculations which have come down to us are charac- 

 terized by great crudeness, but they all show the perpetual tendency 

 of the human mind to ascend from phenomena to their causes. 



The earlier Greek astronomers introduced a system of crystalline 

 transpartnt spheres, revolving one within another, and carrying the 

 planets with them round the earth as a centre. Ear beyond these 

 crystalline planetary spheres was the " primum mobile," the starry 

 sphere, revolving from east to west in twenty-four hours. This system 

 was incorporated into the philoso[)hy of Aristotle, and seems to have 

 commanded the assent of mankind for many ages. Its truth was 

 hardly drawn into doubt till the revival or commencement of true 



