PRESSURE UPON THE SIDES OF VESSELS. 173 



upper part of the spindle, or it will not admit the second nut. The 

 o-lass plate* with its ground side uppermost, is placed between the 

 nuts but between it and the hard metallic surfaces of the nuts small 

 discs of wash-leather are placed, which have been previously moistened 

 with oil or greased with tallow. The leather discs prevent the escape 

 of water between nuts and screw, and protect the glass plate, which 

 without them would probably crack when the nuts are firmly screwed 

 upon it. The object of the leaden weight at the bottom of the spindle 

 is to make the plate hang exactly horizontal. A section of the 

 whole arrangement is given in fig. 124. 



A small scale-pan with short strings will complete the prepara- 

 tions for these experiments. It may be made of a small box such 

 as has been mentioned when speaking of the experiments on the, 

 pulley. The bottom is pierced in the middle, for attaching a small 

 hook of wire to it. \ 



The vessels marked 1, 2, 3, in fig. 120, serve to show 

 the effects of liquid pressure on the walls of vessels. 

 The balance is suspended to the frame (fig. 35) by a 

 hook screwed into the cross-bar, or by a piece of wire : a 

 thread would allow lateral motion. The beam carries a 

 common scale-pan at the right-hand extremity, and at 

 the left a short pan with a hook, from which the vessel 1 

 is suspended with thread. Equilibrium is produced by 

 sand or shot, and one of the unperforated glass plates, 

 held in the fork of the retort-stand by the sealing-wax 

 handle is brought under the cylinder. Care must be 

 taken to place the plate in a perfectly horizontal posi- 

 tion, so that it may touch all round the rim of the 

 suspended vessel. The rim may now be slightly oiled, 

 'to prevent the escape of water, or the vessel may be 

 'pressed more closely against the plate by placing a 

 small weight, not more than 5 81 , in the scale -pan above 

 :he vessel. The whole is shown in fig. 125. 



A plate or basin is now placed below the vessel, and 

 ;he latter filled with water, poured out of a small glass 

 >r a test-tube. That the vessel may be filled in this 



