CHAP, ii.] THE HYDRAULIC OR BRAMAH'S PRESS. 



cavity and against the piston itself, and the closer therefore is 

 the joint. 



The pressure, slight at the commencement of the operation when 

 the substances to be pressed are still not firm, continues to increase 

 until the degree of pressure wished for has been obtained. This 

 result is brought about without the necessity of modifying the force 

 used : the arm of the pump-lever is simply shortened. The pressure 

 primarily depends on the relation of the surfaces of the pistons and 

 on the length of the lever-arm used in the working. Thus the sur- 

 face of the piston p is fifty times that of the "piston p, and the distance 

 from the point H, where the force is exerted, to the point G, on which 

 the lever turns, is ten times larger than GH, the total transmitted 

 pressure is 50 x 10 or 500 times that of the pressure applied. If this 

 equals 100 kilogrammes, the pressure exerted will be 500 100, 

 or 50,000 kilogrammes, allowance being made for loss by friction. 

 Hence it follows that, to diminish this pressure, we only need to 

 lengthen the distance G H, which is easily done by changing the 

 position of the axis G G', on which the lever turns ; by shortening 

 this distance, the pressure is, of course, increased. 



In the present day, the uses of the hydraulic press are very 

 various : it is used to extract the juices of certain plants, such as 

 olives and grapes ; the oil from seeds such as linseed, rape-seed, and 

 castor- seed ; to press paper, stuffs, and forage intended to be sent to 

 great distances, and which, thus compressed, occupy much less space 

 than before the operation ; it is also employed in the manufacture 

 of wax candles, vermicelli, &c. Iron chains and cables for naval 

 use, and girders are submitted to tests in order to prove their resis- 

 tance to strain, and these tests are applied by the hydraulic press. 



The same machine has been employed to raise enormous weights 

 to great heights. In this way the four immense iron-plated tubes 

 forming the gigantic Britannia Bridge, which carries the railway 

 from Chester to Holyhead across the Menai Strait, was raised 

 to the top of the piers. Nearly two millions of kilogrammes were 

 thus raised to a height of thirty-three metres by steam-driven 

 hydraulic presses. 



We may next refer to a recent and very ingenious modification of 

 the first form of the hydraulic press. This form suppresses the force- 

 pump which transmits pressure to the piston of the large press P, 



i) 2 



