326 KANSAS CITY REVIEW OF SCIENCE. 



its superior ; and this grows out of momentum and attraction, like the swinging of 

 the pendulum. For example, whenever the moon gets to either side of the plane 

 of the ecliptic, the tendency of the attractive force, both of the earth, and of the 

 Sun, is to bring it back to that plane. But its momentum causes it to go so 

 far out, before it is overcome by these forces. Then it swings back as the result 

 of the attracting forces. Thus this north and south movement is a sort of oscil- 

 lation. The plane of the ecliptic being to the moon what the gravity-centre is 

 to the pendulum of the clock. In each case we have momentum acting against 

 gravity. Momentum carrying it beyond, attraction bringing it back. And so 

 doubtless in their revolution all the planets oscillate across the orbit of the Sun. As- 

 tronomers must study the Moon in this Hght, learning its real and its relative 

 velocity in all parts of its path, before they can fully master the problem of the 

 causes of its varied movements. And here they may seen farther than has yet 

 been done into the causes of nodal regressions, and of their differences at different 

 times. 



Discovery has not yet reached its limit. Nor has the process of eliminating 

 errors in astronomical science yet been perfected. The coming astronomer has 

 a grand field before him. 



This paragraph is only added because of the dogged persistence I have seen 

 among certain college professors. They are fond of the old illustration of the 

 man walking around some object on a sailing vessel. One especially likes to 

 illustrate the revolution of the earth around the Sun, by a race-horse running in a 

 circular track, making a revolution around a stake at the center. They find in 

 their books much learned bosh, and it is their habit to give it out to their students. 

 How can- it be otherwise when dealing so much with the apparent as though it 

 were real. These illustrations are misleading. The cases are not analogous. 

 To make it analogous, the central stake should have a movement on the surface 

 of the earth, having the same relation to the moving horse, that the movement of 

 the Sun has to the moving earth. Then your race-track would not be circular, 

 but like that given to the earth in these diagrams. Fig. i. But the learned pro- 

 fessor will say, we may ignore all movement except that which is relative. And 

 then, he not only does this, but goes on to ignore the difference between the apparent 

 and the real. If pressed somewhat, he finds shelter under the declaration that 

 "there is no such thing as absolute motion.". Two students. Brown and Jones, 

 having two yards of elastic cord, attempt while walking, to make a revolution, as 

 the moon does around the earth. Brown represents the earth, and walks in the 

 the middle of the road. Jones represelits the moon, and makes the revolution. 

 After walking thirty rods or more, all the time carefully honoring Kepler's laws, 

 with Brown moving on at a steady gait, they have made three-fourths of a revo- 

 lution. Jones says, "how far have I traveled?" Brown answers, "just three- 

 quarters of a revolution in an orbit with a radius of two yards." But Jones wants 

 to know something about movement over the road. Brown tells him that "in 

 astronomy we ignore the road," that "all movement is relative." Jones is not 

 satisfied, he has certain ideas about the ground passed over. But Brown, like a 



