THE j:lements of machinery. 



105 



Thus a -n-eight of 72 pounds may be balanced by four movable pulleys by 

 a T>'eight or power of 9 pounds; with two pulleys, by a power of 13 pounds, 

 with one movable pulley, by a power of 36 pounds. 



These rules apply only to movable pulleys in the same block, when the 

 parts of the rope which sustain the weight are parallel to each. The mechan- 

 ical advantage which the pulley appears to possess in theory, is considerably 

 diminished in practice, owing to the stiffness of the ropes, and the friction of 

 the ropes and wheels. From these causes it is estimated that two thirds of 

 the power is lost. When the parts of the cord are not parallel, the strength 

 of the pulley is very greatly diminished. 



matareCranes J'^' J'""'^ "°^ "^^^i ^IG. 86. 



and Derricks, able pulleys are arranged 

 Tackle and FaU? j^ ^ g^eat variety of 

 forms, but the principla upon which all are 

 constructed is the same. What'is called a 

 " tackle and fall," or " block and tackle,"' 

 is nothing but a pulley. Cranes and 

 derricks are pieces of mechanism usually 

 consisting of combinations of toothed 

 wheels and pulleys, bj^ means of which 

 materials are lifted to different elevations 

 — as goods from vessels to the wharves, 

 building materials from the ground to 

 the stage where the builders are en- 

 gaged, and for similar purposes. One 

 of the most simple forms of movable 

 cranes is represented in Fig. 86. It 

 consists of a strong triangular ladder, at 

 the top of which is a fixed pulley, C, 

 over which the rope attached to the ob- 

 ject to be elevated passes, and is carried 

 down to the cylindrical axle, T, upon 

 which it is wound by means of bars in- 

 serted in holes, or by a crank. This 

 ladder is inclined more or less from the 

 upright position by means of a rope, C 

 D, which is attached to some fixed point 

 at a distance. 



229. The Inclined Plane consists of a hard 

 plane surface, inclined at an angle. 



What is an In- 

 clined Plane ? 



Illustrate the 

 use of an In- 

 clined Plane. 



In Fig. 87, a 6 e repre- 

 sents an inchned plane. 

 230. If wc attempt, for 

 instance, to raise a cask, or any other 

 heavy body into a wagon, we may find 

 that our strength is unequal to lifting it 



a* 



Fig. 87. 



