MOTION, MECHANICS, ETC. 



159 



are to one another, as 2, 3, and 4. Draw c E parallel to 

 A B, and continue A c towards D. 



I take three strings, which are joined in one point by a 

 knot ; and placing the point over c, stretch the strings over 

 c D, c E, c B, and place the pulleys, T, T, T, to coincide Avith 

 the direction of the strings ; then putting the strings over 

 their respective pulleys, at the end of the thread, c D, sus- 

 pend a weight of four pounds, to c E three pounds, and to 

 c B two pounds. These weights will remain in equilibrio 

 while the knot remains over c ; but if it be removed out of 

 that point, they will not be at rest. 



It is evident from this experiment, that power is always 

 lost in the composition of forces ; for here a weight of 

 three pounds, and another of two, only counterpoise a 

 weight of four. 



Screw Press. (Fig. 164.) This represents a small model 

 of a screw, as mounted in a press, extensively used in the 

 arts, as for pressing cotton, for book-binders, etc. 



Price, $2.25. 



Fig. 165. 

 Fig. 164. 



The Endless Screw. (Fig. 165.) This consists of a 

 screw combined with a wheel and axle, in such a manner 

 that the threads of the screw work into the teeth fixed on 

 the periphery of the wheel. Suppose the power applied to 

 the handle of the screw, and the weight attached to the 

 axle of the wheel, then there will be equilibrium when the 

 power is to the weight as the distance between the threads 

 multiplied by the radius of the axle, is to the length of the 

 lever or handle, multiplied by the radius of the wheel. 



Price, in brass, $8.00. 



