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ON THE INSTRUMENTS FROM LEYDEN. 187 



our Lectures. Here is also a little apparatus for demonstrating the 

 properties of the wedge. If 'sGravesande did not make a great deal 

 of important discoveries, he at all events invented an instrument which 

 has rendered great services to science viz., the Heliostat. This is 

 the first which has been constructed. You all know that when a 

 physicist is experimenting upon light he desires that the pencil of 

 light with which he is experimenting should always keep the same 

 direction. Now a pencil of light which comes directly from the 

 sun changes its direction at each moment, and therefore 'sGravesande 

 contrived an apparatus through which the pencils of light fall upon 

 this mirror and are always reflected in the same direction. He also 

 made some experiments which had at that time a very great impor- 

 tance. Scientific men were divided on the question in what manner 

 the power of a body, which has received a certain quantity of velocity, 

 should be calculated. They all agreed on one point, that the power 

 was in the ratio of the masses of the two bodies, but did not agree on 

 the question if the power was in the ratio of the velocity or whether it 

 was in the ratio of the squares of the velocities. 'sGravesande thought 

 that the powers were in proportion to the ratio of the velocities, but he 

 thought it would be very useful to solve the question by direct experi- 

 ment, and the experiments he made were the following. He took two 

 pieces of wet clay and two bodies whose masses were different and 

 those bodies he made fall from different heights on to the clay. 

 Falling on the clay they made holes, and if the power of the two bodies 

 were the same the holes must be the same too. He began by making 

 such arrangements that the mass of each body multiplied by its 

 velocity was the same, and that he could do by allowing them to fall 

 from different heights. They made holes which were different. Then 

 he made them fall from such heights that the mass of the bodies multi- 

 plied by the squares of the velocities were the same and then the holes 

 were the same in each piece of clay. He found thus that he was 

 wrong. But look what a man he was ! His brother-in-law was in the 

 same room in which he was experimenting, and after 'sGravesande 

 had ascertained that in the last experiment the two holes were the 

 same, a shout escaped him so that his brother-in-law came to ask 

 what was the matter. " What is the matter, my dear fellow !" he 

 said, " the matter is, I am quite in the wrong, and Leibnitz is quite in 



