90 WELLS'S NATURAL PHILOSOPHY. 



p gg Fig. 59 represents one of the most 



common forms of the dynamometer, 

 consisting of a band of steel, bent in 

 the middle, so as to have a certain de- 

 gree of flexibility. To the expanded 

 extremity of each limb is fixed an arc 

 of iron, which passes freely through an 

 opening in the other limb, and terminates outside in a hook or ring. One of 

 these arcs is graduated, and represents in pounds the force required j-j^ gjj^, 

 to bring the two limbs nearer together. Thus, if a horse were pulling _^ • 

 a rope attached to a body which he had to move, we may imagine the l) 

 rope to be cut at a certain point, and the two ends attached to the j| 

 ends of the c^rcs, as represented in Fig. 59 ; the force of traction ex- 

 erted by the animal would be seen by the greater or less bringing 

 together of the ends of the instrument. 



In another form of dynamometer, Fig. 60, which is also used as a 

 spring balance in weighing, the force is measured by the collapsing 

 of a steel spring, contained within a cyhndrical case. The construc- 

 tion and operation of this instrument will be easily understood from 

 an examination of the figure. 



What is a Ma- 183. A Machine is an instrument, or 



chine T apparatus, adapted to receive, distribute, ^k 



and apply motion derived from some external force, (9 

 in such a way as to produce a desired result. 



A steam-engine and a water-wheel are examples of machines. They re- 

 ceive the power of steam in the one case, and the power of falling water in 

 the other, and apply it for locomotion, sawing, hammering, etc. 



Do wc produce 184. A MACHINE cau not, under any cir- 

 use"^ of ^ ma! cumstances, create power, or increase the 

 chines? quantity of power, or force, applied to it. 



A machine will enable us to concentrate, or divide, any quantity of force 

 which we may possess, but they no more increase the quantity of force applied 

 than a mill-pond increases the quantity of water flowing in the stream.* 

 _ Machines, in fact, do not increase an applied force, but they 



ehines in reality diminish it, or, in Other words, no macliine ever transmits the 

 diminish force? ^±oIq amount of force imparted to it by the moving poweijf 

 since a part of the power is necessarily expended in overcoming the inertia 

 of matter, the friction of the machinery, and the resistance of the atmosphere. 



• " Power is always a product of nature. God has not vouchsafed to man the means of 

 its primary creation. He finds it in the moving air and the rapid cataract ; in the burn- 

 ing coal and the heaving tide. He transfers it from these to other bodies, and renders it 

 the obedient servant of his will — the patient drudge which, in a thousand ways, adminis- 

 ters to his wants, his convenience, and his luxuries, and enables hira to reserve his own 

 energy for the higber purposes of the development of his mind and the expression of his 

 thoughtB."— Pro/. Henry. 



