20 



THE POPULAR SCIENCE MONTHLY. 



result are a variation in the mass of the hammer-head, and a variation 

 in the length of the handle. By a varied mass there is a varied weight 

 in the hammer; by a varied length of handle there will, with the same 

 muscular effort, be a varied velocity in this mass, and upon a combina- 

 tion of mass and velocity depends the produced energy. Now, if a 

 mass of metal, moving at a known velocity, strike an object, the ener- 

 gy of that blow results entirely from the conditions at the moment 

 of impact. For example, the work in the hammer, IT, as it strikes the 

 nail, JV"(Fig. 20), does not depend upon its velocity through the arc, 



Fig. 20. 



Q JN] but only upon the velocity when commencing contact with the 

 nail. Hence, so long as the material which gives the blow and the 

 mass of it are the same, it is not of any consequence how the velocity 

 was accumulated. It may result from centrifugal or rectilinear action ; 

 it may result from muscular effort, or from steam-pressure, or from 

 gravity. 



It may now be obvious that, other elements remaining unchanged, 

 whatever accelerates the velocity of a hammer increases, according to 

 very clear rules, the energy or power of the same hammer. Hence 

 the tendency of contrivances, as manifested in the addition to steam 

 as well as handicraft hammers ; for example, in the early lift-hammers, 



