1 8 THE POPULAR SCIENCE MONTHLY, 



Before considering the elements upon a combination of which the 

 powers of hand-hammers depend, it will be well to remark upon the 

 circumstances under which this power is actually developed. The 

 development takes place at the instant of contact of the moving 

 hammer with the struck body. Such contacts as those of hammers 



J__= 



=3 



Fig. 19. Tomahawk-Hammer. 



belong to that department of mechanical philosophy called " impact." 

 Impact is pressure of short duration so short that, compared with the 

 time in which the velocity of the impinging body is being acquired, 

 it is inappreciable ; or, if the comparison be between spaces passed 

 through by the hammer-head before impact and during impact, then, 

 generally speaking, the disproportion is the same, and the space passed 

 through after impact is almost inappreciable when compared with the 

 space passed through before impact. 



It may assist in realizing the source as well as the magnitude of 

 the power of a hammer, if the dynamical effect of impact be compared 

 with what may be called the statical effect of pressure. Let any one 

 attempt to drive a nail vertically into an horizontal piece of timber by 

 the statical effect of the simple pressure of a load placed gently on the 

 head, as weights are laid in scale-pans. Let the depth to which the nail 

 is thus moved be measured. Again, let the same nail, under the same 

 circumstances, be driven to the same depth by the impact of a ham- 

 mer-head, then it may for our present purpose be said that the load 

 placed on the nail is a representative statical measure of the impact 

 of the hammer. 



Now, although in any given case the work in a hammer consequent 

 on its mass and velocity may be very great, yet utilizing the whole 

 of the work produced in the expenditure of the accumulated power 

 in the hammer depends upon the resistance met with at the instant 

 of impact. The more perfect this resistance is, the greater will be 

 the value of the work done ; hence the practiee of using massive 

 anvils, firmly fixed, and the necessity for staying all vibrations in the 

 body struck. Let any one attempt to drive a nail in a board not 

 firmly supported, and then by the use of the same means drive a simi- 

 lar nail into the same board supported, and he will appreciate the im- 

 portance of resistance to the progress of a hammer's motion if the full 

 effect of a blow be desired. 



