Chap. XI.

KCI1ANICS.

951

Another method commonly in use is here represented: theshaft to which the machinery is attached can be put out ofmotion by the means of the revolving wheel C. The end of theshaft A is square ; the clutch D is made to slide freely upon itby aid of the handle E E, so that by engaging the revolvingwheel C, which is always in motion, the shaft may be made torevolve. The teeth of the clutch should be made of a sufficientstrength to prevent fracture.

The Bayonet Clutch has its arms A A fixed securely to theshaft, or is cast upon it. These arms have holes drilled throughthem, through which the shanks/,/ of the bayonet pass. Thesliding of the bayonets back upon the shaft puts the whole out ofmotion, and when they are brought together pass through theholes in the opposite wheel, they carry the shaft around withthem. It is perfectly easy by suchmeans to throw the work out ofgear, during the time the mill maybe in motion; but great care shouldbe adopted in throwing them into

gear, as the teeth are subject to be j-Ift —(

broken, particularly when high velo- -■ —cities are employed. They areconsequently only made use of in l|j-jl

Fig 1312.

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Fig. 1314.

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such machines as roiling mills, and ^"/

others where there is a slow velocity.

The shaft on which the cog-wheel is Fig. 1313.

placed, we may suppose engaged ; inturning the machinery we can stop the works by pushing back the shanks// which arefirmly fixed by nuts to the arm D D, and made to slide upon the shaft; around the centralpart, 17 , is a circular groove, like that of a pulley, into which a fork may be introducedwhen it is required to slide the bayonets either forwards or backwards.

The friction clutch is another modification, and whichenables the machinery to be brought into motion more gra-dually. A 15 is the part of the shaft put into motion by themill, and the bayonet is made to slide on the square part ofthe shaft. The bayonet CDE is made to slide on thesquare part of the shaft, or to pass through the arms of a Fcross II I, that is secured firmly to the shaft; F G is the partof the shaft to be connected, and upon it is fastened a drumor pulley, with a hook on the outside, and which is made topress gently upon the drum by screws. When the machineis required to be put in motion, the hoop LM is turnedround by the bayonet clutch, and by the friction of the hoop Fig - 1315.

L M upon the drum, a gradual and easy action is obtained. This simple and ingeniouscontrivance is applicable to large machines, and particularly where any shock or strain givento the works would be injurious.

Friction destroys, but does not generate motion, it is therefore unlike gravity, or anyother force which, while retarding motion in one direction, accelerates it in another. Theforce of friction may be said to destroy the law of continuity and motion, and it is of theutmost importance that every means should be adopted to diminish its influence in practicalmechanics as much as possible. If power be applied to a machine of any kind, an equi-librium takes place when the velocity of the power is to the velocity of the resistance, asthe weight is to the power; in this state the machine may be considered at rest, andincapable of performing any work. If we increase the power, the machine will move withmore and more velocity, its motion gradually increasing as long as the power exceeds theresistance; but should the power be lessened, or the resistance increased, the motion of themachine will be diminished until it arrive at a state of uniform motion. This increase ofresistance may be produced by an increase of friction, which generally accompanies anincreased velocity, or by the resistance of the air, which is always greater as the velocityaugments. The power of a machine is therefore diminished by an increase of velocity, andfrom this cause it is that machines acquire a uniform motion. The force which opposesthe production of motion in a machine is the resistance, and the chief of that to be over-come arises from the work to be performed ; but there is, in addition, the resistance fromfriction, and from the inertia of the parts of the machinery, a considerable quantity of themoving power being employed to overcome the first obstacles to motion. An axle with awheel at each extremity, moved by two currents of water running with different velocities,has at first a slow motion, but if the fall of either be increased, the work will be performed

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