262 



THE LEVER AND WHEELWOBK. 



which a chain or rope is coiled. The chain passes over a pulley, D, at the 

 top of the gib. At the end of the chain a hook, O, is attached, to support the 

 weight W. During the elevation of the weight, it is convenient that its recoil 

 should be hindered in case of any occasional suspension of the power. This 

 is accomplished by a ratchet-wheel attached to the barrel M, as illustrated in 

 fig. 10 ; but when the weight W is to be lowered, the catch must be removed 

 from this ratchet-wheel. In this case, the too-rapid descent of the weight is 

 in some cases checked by pressure excited on some part of the wheelwork, so 

 as to produce sufficient friction to retard the descent in any required degree, or 

 even to suspend it, if necessary. The vertical beam at B resting on a pivot, 

 and being fixed between rollers, allows the gib to be.turned round in any direc- 

 tion ; so that a weight raised from one side of the crane may be carried round 

 and deposited on another side, at any distance within the range of the gib. 

 Thus, if a crane be placed upon a wharf near a vessel, weights may be raised, 

 and, when elevated, the gib may be turned round so as to let them descend 

 into the hold. 



The power of this machine may be computed upon the principles already 

 explained. The magnitude of the circle, in which the power at I moves, may 

 be determined by the radius of the winch, and therefore the number of teeth 

 which a wheel of that size would carry may be found. In like manner, we 

 may determine the number of leaves in a pinion whose magnitude would be 

 equal to the barrel M. Let the first number be multiplied by the number of 

 teeth in the wheel K, and that product by the number of teeth in the wheel L. 

 Next, let a number of leaves in the pinion H be multiplied by the number of 

 leaves in the pinion attached to the axle of the wheel K, and let that product 

 be multiplied by the number of leaves in a pinion whose diameter is equal to 

 that of the barrel M. These two products will express the power of the ma- 

 chine. 



Toothed wheels are of three kinds, distinguished by the position which the 

 teeth bear with respect to the axis of the wheel. When they are raised upon 

 the edge of the wheel, as in fig. 27, they are called spur wheels, or spur gear. 

 When they are raised parallel to the axis, as in fig. 29, it is called a crown 



Fig. 29. 



wheel. When the teeth are raised on a surface inclined to the plane of the 

 wheel, as in fig. 30, they are called bevelled wheels. 



Fig. 30. 



If a motion round one axis is to be communicated to another axis parallel to 



