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THREE:It is a strange thought that with all the order, system, and regularity, existing in nature, there is nothing but the movements of the heavenly bodies constant enough to form a base for gauging tests. The French standard based upon the calculated length of the meridian may be traced to this source.The sudden and varied resistance to line shafts tends to loosen couplings, destroy gearing, and produce sudden strains that are unknown in other cases; and shafting arranged with the usual proportions for transmitting power will soon fail if applied to driving trip-hammers. Rigid connections or metal attachments ace impracticable, and a slipping belt arranged so as to have the tension varied at will is the usual and almost the only successful means of transmitting power to hammers. The motion of trip-hammers is a curious problem; a head and die weighing, together with the irons for attaching them, one hundred pounds, will, with a helve eight feet long, strike from two to three hundred blows a minute. This speed exceeds anything that could be attained by a direct reciprocal motion given to the hammer-head by a crank, and far exceeds any rate of speed that would be assumed from theoretical inference. The hammer-helve being of wood, is elastic, and acts like a vibrating spring, its vibrations keeping in unison with the speed of the tripping points. The whole machine, in fact, must be constructed upon a principle of elasticity throughout, and in this regard stands as an exception to almost every other known machine. The framing for supporting the trunnions, which one without experience would suppose should be very rigid and solid, is found to answer best when composed of timber, and still better when this timber is laid up in a manner that allows the structure to spring and [107] yield. Starting at the dies, and following back through the details of a trip-hammer to the driving power, the apprentice may note how many parts contribute to this principle of elasticity: First��the wooden helve, both in front of and behind the trunnion; next��the trunnion bar, which is usually a flat section mounted on pivot points; third��the elasticity of the framing called the 'husk,' and finally the frictional belt. This will convey an idea of the elasticity required in connecting the hammer-head with the driving power, a matter to be borne in mind, as it will be again referred to.