THE TOGGLE OR KNEE JOINT. 



317 



This joint is shown in the illustration. It consists of two 

 levers, jointed together at one end, and having the other ends 

 jointed to the objects which are to be pressed asunder. It will 

 be seen that if the centre of the Toggle be pushed or pulled in 

 the direction of the arrow, so as to straighten the levers, the 

 amount of pressure upon them is enormous. Such an appa- 

 ratus as this combines simplicity and power in a wonderful 

 manner, and is greatly used in machinery, especially in 

 presses, where the force is required to be great, but not of long 

 duration. 



An ordinary two-foot rule, when bent, affords a good 

 example of the Toggle-joint, and will exert a wonderful 

 amount of force. 



The illustration represents one of the common printing- 

 presses that are worked by hand. When the workman draws 



PP.INTTOG-PBESS. 



the handle horizontally, he causes the two portions of the 

 Toggle to approach a straight line. The upper half of the 

 Toggle being jointed to the fixed beam above, and the other 

 half to the movable plate or " platen " below, it is evident that 

 the latter will be pressed downwards with enormous force. 

 Indeed, so great is the power of this instrument, that a man 

 of moderate strength can exert a pressure of many tons. 



WE now proceed from Art to Nature, and take first the 

 human knee, being the joint from which this piece of mecha- 

 nism has derived one of its names. 



If the reader will look at the figure of the fencers, he will 

 see that the arm and leg are both Toggle-joints. In the one 



