GENERAL SCIENCE 



Belts and belt wheels are used for changing the speed 

 of machinery and for the transmission of energy over a 

 considerable distance so that it can be utilized in another 

 place. 



Very complex or compound machines can be made by 

 combining two or more of the simple machines. The 

 mechanical advantage of a compound machine is equal 

 to the product obtained by multiplying together all the 

 mechanical advantages of the sim- 

 ple machines included in the struc- 

 ture of the compound one. By 

 such combinations a very high 

 mechanical advantage can be ob- 

 tained. Derricks or travelling 

 cranes used by railroads are usually 

 combinations of block and tackle, 

 cogwheels, and a windlass on which 

 to wind the rope. The machinery 

 in factories, printing establish- 

 ments, and on Western farms is 

 very complex, and these machines 

 require a considerable amount of 

 knowledge on the part of the per- 

 Every successful farmer of today 

 must be more or less of a machinist. 



130. Power. Review energy, force, and work in 117 

 of this chapter. Power is the rate at which work is done, 

 or it is the time rate of doing work. The word work 

 does not include time. It requires just as much work 

 to carry 200 bricks to the top of a 20-foot building in two 

 days as it does to carry them up in two hours. We do 

 just as much work when we walk to the top of a hill as 

 when we run up. The force which we exert is* much 



HAND DERRICK 

 With hoisting tackle of 

 rope and pulleys. 



sons operating them. 



