312 On Printing Presses and their Theory. 



summer, where the spindle was mserted, was attached a 

 species of inclined plane, rounded off so as to have a varia- 

 ble inclination. Through the spindle immediately beneath 

 ran a cross bar, which plied against this winding surface, 

 and forced the spindle down as it was turned round ; rapidly 

 at first, but more slowly as the inclination diminished, and 

 at last with a velocity as trifling as was shewn by experi- 

 ence to put the press into the best working state. In a 

 press recently invented by Mr. Medhurst, an ingenious Eng- 

 lishman, the power is gained by means of two iron rods, 

 one on each side of the spindle. These rods pass down 

 from the summer to the top surface of a circular enlarge- 

 ment of the spindle, and rest at each end in hollows which 

 allow them a racking motion. The two extremities of each 

 are equidistant from the centre of the spindle, but are pla- 

 ced in a winding position when the platen is raised^ The 

 bar turns the spindle partly round, and moves the lower 

 ends of the rods so that they come towards a vertical posi- 

 tion, and bear down the platen with that kind of increasing 

 mechanical power which every one has seen exemplified in 

 bringing a prop erect by driving at right angles against the 

 bottom.* 



But in most of the recent attempts to improve the con- 

 struction of the printing press, a kind of mechanical power 

 has been resorted to in different forms, somewhat different 

 from either of the foregoing ; one which is well known to 

 every theoretical and practical mechanician, but which has 

 scarcely acquired a distinct name. To attempt to reduce it 

 to the head of the lever or wedge, as has been sometimes 

 done, appears an unwarrantable extension of the meaning 

 of these terms ; and 3^et I know not how to designate the 

 principle better than to call it that of combined levers. It is 

 the power of thrusting possessed by the outer extremities of 

 two straight rods, placed obhquely end to end or riveted to- 

 gether, when the moving force is apphed to bring them 

 straight. Let a pair of compasses, or a carpenter's rule the 

 two halves of which shut together with a joint, be opened 

 nearly straight and placed between two parallel obstacles : 

 on taking the rivet between the thumb and finger and vary- 

 ing the angle, it will require no skill in mechanics to discov- 



* The principle of thi« comhination will be investigated in the Supple- 

 mfnt: 



