Foot-Pound and Horse-Power. 27 



A thermometer may be correct at the freezing and boil- 

 ing points and inaccurate at most intervening degrees, 

 growing out of the unequal diameter of the tube in differ- 

 ent portions and the fact that all degree marks may be 

 made of the same length. Errors of this sort can be de- 

 tected only by comparing the thermometer with a standard. 



35. Units of Work and Energy. It has been found neces- 

 sary in dealing with the numerical relations of work and 

 energy to adopt standards of measurement just as has been 

 done for lengths, volumes, surfaces and mass, and various 

 units are in use. 



36. Foot-pound and Foot-ton. A common unit of work 

 is the foot-pound, which is a mass or weight of one pound 

 lifted vertically against or in opposition to the force of 

 gravity. 



If a body is moved one foot in any other direction than 

 against the force of gravity and the intensity of the pull 

 or push necessary to do this is equal to that required to lift 

 one pound, then in this case the work done is one foot- 

 pound. If 2,000 pounds is lifted one foot high then 2,000 

 foot-pounds of work have been done, and this is sometimes 

 designated a foot-ton. The same intensity of pull in any 

 other direction may be expressed in the same terms. 



Time is not a factor taken into account in simply ex- 

 pressing the amount of work done for the reason that a 

 very small force when permitted to act for a very long 

 time may raise the same weight through one foot, which 

 would require a very intense force if permitted to act but 

 a very short time. 



37. Horse-power. When the rate at which work is done 

 and the intensity of the force required to do the work at 

 the stated rate are to be expressed qualitatively, then a 

 unit involving time must be chosen and the horse-power 

 is one of these. The horse-power used by English and 

 American engineers is the amount of energy which can 

 do 550 foot-pounds of work per second or 33,000 foot- 



