﻿illustrate the Laws of Motion. 337 



the bell are all included in a battery of a few cells. The pendu- 

 lum being; held to one side (for in its position of rest the point 

 keeping down the saddle breaks the current), the current passes. 

 An iron ball is attached to the electromagnet, which is then 

 hoisted to the height of sixteen feet (as pointed out by the scale) 

 above the surface of a cushion on which the ball is to fall, so as 

 to deaden the sound. The eyes of the audience are to be directed 

 to the cushion, while their ears listen for the bell. When the 

 pendulum is released, the first break rings the bell and drops the 

 ball together ; the second break rings the bell again, and, as 

 nearly as the eye and ear can judge of simultaneity, identically 

 at the same moment as the ball reaches the cushion. If the ball 

 be hoisted to the height of eighteen feet, it is seen to be too late ; 

 if lowered to fourteen feet, it is seen to be too soon. From this 

 may be inferred the amount of accuracy of which the experiment 

 is capable. It must be remembered that the arrangement is one 

 for illustrating a certain quantitative result to an audience, and 

 not for determining in the cabinet an important natural constant. 



The next experiment may perhaps claim greater accuracy than 

 that just described, as there is always more or less of difficulty 

 (more, apparently, with some persons than others) in perceiving 

 the identity of time of two phenomena presented to two different 

 senses simultaneously. This arrangement depends only on the 

 sense of sight ; and the law which it demonstrates may be thus 

 enunciated. 



"A body projected in a horizontal direction with any velocity 

 whatever, will take the same time to reach the ground as a body 

 let fall vertically through the same height/' 



This clearly is an important truth to impress on a beginner 

 endeavouring to understand the second law of motion. It will 

 teach him that, in at all events one very important case, the 

 effect of the same force acting for the same time does not in the 

 least depend upon the circumstance as to whether the body on 

 which it acts is at rest or in motion ; and perhaps this is the 

 least self-evident of the truths that are wrapped up in Newton's 

 concise enunciation of his law. 



The arrangement consists of two parts, and the assistance of 

 electricity as a rapid messenger is again called in. ABCD 

 (fig. 3) is a piece of wood 1 J inch thick ; AC is an arc of a 

 circle of two feet radius, the tangent to which at C is horizontal, 

 A C being about a foot long. A ball being intended to run 

 down this piece, it is grooved ; and A C is one of the edges of the 

 groove, the other being of course at a distance of 1J inch on the 

 other side of the wood. The ball which rolls down the groove 

 is a sphere of %\ inches in diameter, made of wood and neatly 

 covered over with tinfoil. The two edges of the groove are each 



Phil. Mag. S. 4. Vol. 37. No. 250. May 1869. Z 



