Page:Popular Science Monthly Volume 9.djvu/36

18 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



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 hammer-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 practice 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 similar nail into the same board supported, and he will appreciate the importance of resistance to the progress of a hammer's motion if the full effect of a blow be desired.