CHAMBERS'S INFORMATION FOR THE PEOPLE. 



elbow-joint ; the power is the strong muscle called 

 the biceps, which passes down the front of the 

 humerus, and is attached at A to the radius, one 

 of the two parallel bones composing the forearm ; 



Fig. 12. 



the resistance is the weight of the forearm, to- 

 gether with anything held in the hand, the two 

 being supposed combined in one weight acting at 

 H. In this way, the contraction of the muscle to 

 the extent of an inch will raise the hand through 

 an arc of twenty inches. It will be readily seen, 

 that as the angle at the elbow-joint is enlarged, 

 the tendon of the muscle pulls more obliquely to 

 the bone of the forearm, and thus acts at an in- 

 creasing disadvantage; hence the arm held out 

 straight has comparatively little power to sustain 

 a weight 



Combinations of Levers. 



By disposing a number of simple levers, so that 

 the one shall act on the other, the effect of the 

 power may be increased to any desired degree. 

 Fig. 13 represents a combination of three levers of 



E r__p 



I 



Fig. 13- 



the first kind. They are supposed, for simplicity, 

 to be of equal length, the long arms being six 

 inches each, and the short ones two inches each ; 

 required the weight which a moving power of 

 I pound at P will balance at W. In the first 

 place, i pound at P would balance 3 pounds at E, 

 according to the rule of calculation for the simple 

 lever ; for I X 6 = 3 X 2. This upward pressure 

 of 3 pounds at E is next converted, by the second 

 lever, into a downward pressure at D three times 

 as great, or equal to 9 pounds ; and, lastly, the 

 downward pressure of 9 pounds at D is converted, 

 by the third lever, into an upward pull at W of 3 

 times 9, or 27 pounds. Thus, i pound at P will 

 balance 27 pounds at W. 



The general rule for combinations of levers, 

 whether equal or not, is, that the power multiplied 

 successively by all the arms next it in the system, 

 is equal to the weight multiplied by all the other 

 arms. From this equation we may find what 

 212 



power will be necessary to raise any given weight, 

 or what weight any given power will raise. 



Machines for weighing loaded wagons, frequently 

 seen at toll-bars, consist of a system of levers 

 arranged below a table or platform on the level of 

 the road. The wagon being wheeled upon the 

 platform, is balanced by a comparatively small 

 weight attached to the extremity of the system. 

 The balance of Quintenz, much used for weigh- 

 ing luggage at railway-stations, is an ingeniously 

 arranged combination of levers. The principle of 

 the steelyard is frequently superadded to a com- 

 bination of levers in these weighing-machines, so 

 that one counterpoise serves for all cases. 



So long as a force acts perpendicularly to the 

 straight arm of a lever, its effort to turn the lever 

 about the centre is the same, whatever angle that 

 arm makes with the other arm. If the equal 

 forces, P, Q, act perpendicularly to the equal 

 arms CA, CB, of the bent lever ACB (fig. 14), they 

 will keep each other 

 in equilibrium on the 

 same grounds as when 

 the lever is supposed 

 straight ; namely, that Q, 

 there is no cause why 

 the one should prevail 

 over the other. And 

 if we suppose the arm 

 CA removed and again 



Fig. 14. 



fixed in the position CA', with an equal force P' 

 acting in the perpendicular line A'P', the force Q 

 will still be balanced as before ; that is, the effort 

 of the force P to turn the lever is the same in its 

 new position as it was in its old. 



Suppose, now, that two unequal forces, P and Q 

 (fig. 15, a), hold one another in equilibrium on the 



Pv 



Fig. 15- 



unequal arms AC and CB, and let the arm CB 

 take the position shewn in b, the effort of Q to 

 turn the lever will not be altered by its new posi- 

 tion any more than in the case of the equal arms ; 

 so that Q and P still balance each other. It is 

 thus of no consequence what angle the two arms 

 of a lever make with each other, so long as the 

 forces act at right angles to the arms ; P X AC is 

 still equal to Q X BC. 



But when a force acts obliquely to its arm, part 



Fig. 1 6. 



of it is wasted in pulling or pushing against the 

 fixed centre, and the virtual or effectual leverage 



