58 CONSTRUCTION OF THE APPARATUS. 



experimenting, however, the weights should not be allowed to come 

 in actual contact with the floor, for this would bend the hooks or even 

 break them off. A small box filled with sawdust placed upon the spot 

 which the descending weight would touch will protect it against 

 damage, in case the timely stopping of it should have been forgotten. 

 The pendulum consists of a disc of lead, about l k r in weight, 

 suspended by a fine silken cord. Procure from the turner a wooden 

 disc, from 6 to 8 cm in diameter, and about 2 cm thick ; make in 

 it a small hole, but not quite through, and insert into it a wire 

 as in fig. 46, surround the disc with paper, place it as horizontal 

 as possible, and pour into it about l k r of lead, rather more 

 than less. The lower piece of wire which projects is turned 

 into a small ring, which is close to the disc, the upper wire is 

 allowed to project a few centimetres, made quite straight and 

 perpendicular to the disc, and a small hook is bent at its ex- 

 tremity. The small weight is made- of a flat piece of brass or 

 copper, about l cm in diameter and O 11 ^ thick, to the middle of 

 which a small wire ring is soldered. The small weight must be 

 perfectly horizontal when suspended by the thread, so as to lie 

 quite flat upon its support (fig. 44), for which a china plate may be 

 used. The thread which carries the leaden disc should be so long 

 that the whole of its length measured from the point of suspension, 

 that is, from the lower edge of the arms of the retort-stand which 

 holds it, together with, the length of the projecting wire, and 

 half the thickness of the leaden disc, should be 99 cm . The distance 

 should of course be measured while the disc is suspended and the 

 thread stretched by its weight. The fine thread for the small 

 weight is made just so long as to allow the weight to rest upon the 

 plate when the pendulum is at rest ; the small glass tube through 

 which the thread passes should be about 2 nun wide, with rounded 

 edges, to prevent the thread from being cut. In using the pendulum 

 the heavy weight is moved aside from its position of equilibrium 

 until the small weight is raised not quite so high as the glass tube, 

 and is then let go. If the above instructions for adjusting the 

 weights and distances have been strictly carried out, the pendulum 

 will beat seconds, with an exactness quite sufficient for our pur- 

 pose ; it will not vibrate very long but quite long enough for one 

 experiment, and may be started again for the next one, A pendu- 

 lum properly constructed for precisely beating seconds and going 

 for some time is expensive, and not necessary for our experiments. 



10. Motion of Falling Bodies, Bodies allowed to 

 fall from different heights acquire different velocities. 



