MOTION, MECHANICS, ETC. 15 7 



the cord will balance the carriage, weighing four ounces, 

 and a small additional weight will draw it up. 



The Wedge. To the frame there are hinged two maho- 

 gany boards, which are drawn together by a brass frame- 

 work, to which cords are attached, which pass over and are 

 supported by pulleys, having weights at the lower ends of 

 the cords. Rollers are also fixed in this frame-work, against 

 which a wedge is introduced, having a cord and weight at- 

 tached, which weight, with that of the wedge, is to be con- 

 sidered as that of the impelling power. The weights at- 

 tached to the cords and supported over the pulleys, are to 

 be considered as the resistance of the wood acting equally 

 against the opposite sides of the wedge. If the back of the 

 wedge, D, is of the same length as each of the sides, the 

 wedge, with a weight sufficient to make it equal ten ounces, 

 will be in equilibrio against ten ounces, suspended from each 

 of the cords, B and c. If the back of the wedge, E, be equal to 

 one half the length of each of the sides, it will be in equilibrio 

 with twenty ounces from each of the cords, B and c, or the 

 power is to the weight as the back to the sum of the sides. 



Fig. 162. 



The Screw (Fig. 162, as above) consists of two parts, a 

 solid cylinder, around the surface of which passes what is 

 called the thread of the screw, and a corresponding cylin- 

 drical cavity. 



One part is commonly fixed, whilst the other is turned 

 round ; and in each revolution the movable part is carried 

 in the direction of the cylinder, through a space equal in 

 length to the interval between t\vo contiguous threads. A 

 screw is seldom used without the application of a lever to 

 assist in turning it ; consequently, as much as the circumfer- 

 ence of a circle, described by this handle or winch, is greater 

 than the intervals or distance between the spirals, so much 



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