MECHANICS. 



ration of weight it is designed that the 

 divisions should indicate ; and fie suc- 

 cessive positions of the index will deter- 

 mine the divisions. 



A method of graduating the arch 

 may be derived from mathematical 

 investigation, independently of experi- 

 ment, but we cannot introduce it here. 



(48.) We shall conclude this chapter 

 with the description of a more modern 

 instrument, somewhat similar to the 

 bent lever balance. Brady's balance is 

 represented in Jig. 30. BCD is an 



Fig. 30. 



iron frame, being much thicker and 

 heavier towards B than in the other 

 parts. It is supported upon a fulcrum 

 G, and F is a moveable suspender to 

 which a scale or hook is attached to 

 receive the substance to be weighed. 

 There are three distinct positions, H 

 F I, in which this suspender may 

 be placed, and for which there are 

 three distinct graduated scales. When 

 a weight is suspended from the hook, 

 or placed in the dish, the machine is 

 turned round the fulcrum G, and the 

 side C descends until equilibrium is 

 established, and the weight, is read off 

 from that scale which corresponds to 

 the position of the suspender. 



One scale shows the weights of bodies 

 not exceeding two pounds, and is gra- 

 duated to ounces ; another determines 

 weights not exceeding eleven pounds, 

 and is Graduated to t\vo ounces ; and 

 the third determines weights not ex- 

 ceeding thirty pounds, and is graduated 

 also to four ounces. 



CHAPTER VI. The Axle in the Wheel 

 Methods of applying the Power 

 Windlass Capstan Pressure on 

 the Pivots Dejects of this Machine- 

 Methods of removing them Method 

 of Regulating the Variation of a 

 Power or Resistance. 



(49.) WHEN a very severe force, act- 

 ing through a small space, is required 

 to be produced by a comparatively small 

 power, the common lever, whether simple 

 or complex, is a machine well adapted 

 to produce the effect. But the defect 

 of this ensine is, that, under all circum- 

 stances, it works through a very limited 

 space, and that the action of the power 

 is almost necessarily intermitting. Thus 

 if a weight is to be raised by a lever, 

 the prop or fulcrum is generally placed 

 somewhat above the point at which the 

 working end of the lever is to be applied 

 to the weight. The end to which the 

 power is applied is, therefore, above the 

 fulcrum. On depressing that extremity, 

 the weight is raised until the end at 

 which the power is applied is a little 

 below, and that which sustains the 

 weight a little above the fulcrum. The 

 weight must then be supported by some 

 other means until the fulcrum be raised 

 above it, and another similar exertion 

 of the power be made. 



(50.) To remove this inconvenience 

 a modification of the lever, called the 

 axle in the wheel, is used. This ma- 



F'ff.31. 



chine consists of a cylinder A C, and a 

 wheel B D, having the same axis, at the 

 extremities of which are points on which 

 the whole may turn. 



Fig. 31 is a section of this machine, 

 taken in the plane of the wheel ; and 

 Jig. 32 is a projection on a plane at 

 right angles to the wheel. The power 

 P is applied at the circumference of the 

 wheel, and generally in the direction, 

 B P of the tangent. A cord is wrapped 



