PRINCIPLES OF MACHINERY. 87 



erroneous notion that, in some way or other, a 

 machine performs worlc of itself that it is a source of 

 power. It were as reasonable to imagine that a bank is a 

 source of real money. The bank can pay out no more 

 than it receives ; neither can a machine. A man may go 

 to the bank with a ten-dollar gold piece, and get for 

 it ten one-dollar gold pieces. In like manner, he may go 

 to a machine with an ability of moving ten pounds one 

 foot in a given time, and get for it the ability of moving 

 one pound ten feet in the same time. He may exchange 

 what he has for what he prefers ; but, in the case of the 

 bank and of the machine alike, the equivalent must be 

 paid, and generally a commission for the transfer. 



166. Of what Use are Machines? Some of the 

 many advantages resulting from the use of machines are : 

 (1.) It enables us to exchange intensity for a velocity 



otherwise unattainable, as in the case of the sewing 

 machine or spinning wheel. 



(2.) It enables us to exchange velocity for an intensity of 

 power otherwise unattainable, as in the case of lift- 

 ing a large stone with a crow-bar or pulleys. 



(3.) It enables us to change the direction of our force, as 

 in the case of hoisting a flag on a flag-staff. It 

 would be inconvenient to climb the pole and then 

 draw up the flag. 



(4.) It enables us to employ other forces than our own, as 

 the strength of animals, the forces of wind, water, 

 steam, etc. 



167. General Laws of Machines. The work to 

 be done by a machine is generally called the weight or 

 load. The work of the power (e.g., foot-pounds) is always 



