230 PROFESSOR TAIT ON IMPACT. 



These, of course, involved an amount of work which, though not perhaps more difficult 

 than the rest, was incomparably longer and more tiresome. 



Description of the Apparatus. 



Two beams nearly 12 feet long, and 6 inches by 2\ inches cross section, are 

 rigidly fixed, vertically, and at a distance of 8^ inches from each other, to a massive 

 stone pillar. To them the rails, which act as guides for the falling body, are screwed, the 

 distance between them being 6§ inches. At the base, between the rails, is a cylinder of 

 lead, 6 inches by 6 inches, firmly imbedded in a mass of concrete, and having on its 

 upper end a hole, f inch deep and 1^ inch diameter, for holding the lower end of the 

 substance experimented on. This consists of cork, india-rubber, vulcanite, &c, as the 

 case may be, cut into a cylinder, 1^ inch diameter, and 1^ inch long, with the lower 

 end flat and the upper slightly rounded. It thus projects about |- inch after being 

 thrust home into the hole in the leaden cylinder, in which it rests on a thin disc of gutta- 

 percha. This was found effectually to prevent the cylinder's being displaced in the lead- 

 block. Before it was introduced, the cylinder was occasionally left not in contact with 

 the bottom of the hole, so that the record of the next impact was vitiated. Sometimes, 

 indeed, the cylinder had jumped entirely out of the hole before the block redescended. 



In a plane, parallel to that which contains the guides and nearly 1\ inches from it, a 

 massive fly-wheel, 28f inches diameter, whose moment of inertia is 102 "6 in lbs. sq. ft., 

 is placed. The iron frame supporting it is fixed to the concrete floor by means of bolts, 

 so that the whole can be rigidly fixed in position or lifted back at pleasure. A thick 

 wooden board is firmly attached to the front of this wheel, and on it is laid a .sheet of 

 felt. On the top of the felt, an octagonal plate of glass, about § inch thick, the edges of 

 which are bevelled, is placed, and then firmly pressed to the board by means of bevelled 

 metal plates, covered with felt, and screwed down on four alternate edges. 



The mass of the glass is 28 lbs., its moment of inertia 2 5 "21. For the wood these 

 are 21'5 and 24*19 respectively. The total mass (including the fly-wheel) being 1225 

 lbs., ¥ is found to be about 1*24 sq. ft. 



A rope passing up the outside of one of the beams, over two small pulleys, and down 

 between the rails, serves to raise and lower the block, next to be described, or to keep it 

 suspended by a hook at any desired height. A cord running parallel to the rope is 

 attached to the catch of the hook at the end of the rope, so that by pulling this cord the 

 hook is tilted and allows the block to fall. 



The block is rectangular, and formed of hard wood (plane-tree along the grain), \l\ 

 by 7\ by 1\ inches, weighing b\ lbs. Down the centre of each of the edges runs a deep 

 groove, at the ends of which pieces of iron with a polished groove of U section are screwed 

 on. It is on these that the guides bear while the block is falling. The guides and Us 

 being well oiled, the friction is reduced to a minimum. 



A brass plate, 5^ inches by 2^ inches, is sunk into the face of the block about \ inch, 



