MECHANICAL WORK. 75 



tie a piece of thread to the head of the pin, and wind the other ex- 

 tremity round a lead pencil close to the pointed end, making the 

 distance between the pin a and the point of the pencil 40 cm . Now 

 describe the arc e f g with the point of the pencil. Next insert 

 successively an easily fitting pin into the holes 6, c, d, and de- 

 scribe the arcs fh, fi, fJc, by placing the thread which remains fixed 

 at a upon the left side of the pin, so that it successively assumes 

 the positions abh, aci, adk. The pencil must always be kept hori- 

 zontal and perpendicular to the plane of the cardboard. The pen- 

 cilled arcs may then be carefully drawn in ink, using the latter 

 sparingly if the cardboard is apt to make the ink run. The pencil 

 is now replaced by a small but heavy weight, best by a leaden bullet 

 with a small hook attached. The end of a piece of wire 3 cm long 

 and l mm thick is somewhat bent and introduced by that end into a 

 bullet mould, the other end slightly projecting from the mould. 

 Lead being poured in, the whole is allowed to cool, and the super- 

 fluous lead round the wire carefully cut away, so as not to cut the 

 wire itself, which is then bent into a little hook. The bent end of 

 the wire must be made larger than the aperture of the mould ; other- 

 wise it would be pushed out by the lead which is poured into the 

 mould, because iron, and also copper and brass, swim upon liquid lead. 

 This precaution is of course unnecessary if the wire be held firmly 

 in the mould while the lead is being poured in. After the bullet is 

 tied to the thread, its centre should be exactly opposite to the point 

 f, when the thread hangs vertically. 



'he moveable pin being removed, the bullet is 

 raised to e, and allowed to fall. Drawn downwards 

 iby gravity, but unable to fall perpendicularly, it is 

 forced by the thread to describe the arc ef. The 

 velocity acquired is just sufficient to move the bullet on 

 the other side upwards to </, or, more precisely, nearly 

 to g- because a small amount of work is expended in 

 overcoming the resistance of the air and the stiffness of 

 lie thread. Further, if the moveable pin is again in- 

 serted into b, the thread placed upon the left side of it, 

 md the bullet as shown in the figure brought to A, the 

 trc hf will be described by the bullet if left to itself. The 



