﻿illustrate the Laws of Motion. 339 



For demonstrating the particular point in question, this arrange- 

 ment apparently leaves little to be desired. 



"The path of a projectile is a parabola." The method of de- 

 monstrating this is indicated in M. Daguin's Traite de Physique 

 (vol. i. p. 94). The arrangement for the lecture-room based 

 upon it is simple enough. A quadrant (fig. 4) of two feet ra- 

 dius is made of wood 1^ inch thick, 

 and grooved as in the preceding ex- Fig-. 4. 



periment. This is to be very firmly 

 supported parallel to the wall and 

 about two inches distant from it, and 

 six or seven feet from the ground. 

 Down this a wooden ball 2J- inches 

 is to roll, and it is proposed to prove 

 that the path it follows after leaving 

 the groove is a parabola. That the 

 ball after rolling down shall describe 

 precisely the same path each time, it 

 is necessary that the direction of projection be perfectly constant; 

 this is ensured by fixing the quadrant very firmly; and then the 

 direction of projection is the tangent at the base, which for con- 

 venience is horizontal, or nearly so. It is no less necessary that 

 the velocity of projection be constant ; this is provided for by 

 allowing the ball always to start from the same position on the 

 quadrant. To secure this, a small ledge is fastened at the top, 

 and the ball is, before each descent, brought home against the 

 ledge, and thence allowed to fall. By this means the same 

 trajectory can be reproduced as often as desired. 



To show that this is a parabola, the following simple plan is 

 employed. A number of little arches are made from slips of 

 cardboard 1 inch wide ; these arches are about 4 inches across and 

 b inches high, something the shape of the letter U turned upside 

 down. They are fastened to the wall by drawing-pins or other- 

 wise all along the constant path traversed by the ball, as shown 

 m the figure. The mode of placing them is easy. First, one 

 is arranged so that after each descent the ball goes through its 

 centre; then the next is similarly placed, and so on until ten or 

 thereabouts have been affixed, through all of which, and without 

 touching any, the ball will pass, after leaving the curve, finally 

 tailing into a basket placed to catch it. 



* By joining the centres of the arches along the wall by a curve 

 the position of the focus and directrix of the parabola will be 

 easily found, and the nature of the trajectory consequently de- 

 monstrated. "L J 

 Royal College of Science for Ireland, 

 Stephen's Green, Dublin, 

 March 25, 1869. 



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