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THE POPULAR SCIENCE MONTHLY. 



distance, and therefore I have here a large front wheel alone. On 

 moving this along the table, any twisting of its direction instantly 

 causes it to deviate from its straight path ; now, suppose I do not let 

 it deviate, but compel it to go straight, then at once a great strain is 

 put upon the table, which is urged the other way. If the table can 

 move, it will instantly do so. A table on rollers is inconvenient as an 



Fig. 3. 



instrument, let us therefore roll it round into a roller, then on moving 

 the wheel along it the roller will turn and the amount by which it turns 

 will correspond to the height of the second figure drawn by the cart 

 integrator. If, therefore, the wheel is inclined by a magnet under the 

 influence of an electric current, or by any other cause, the whole amount 

 of which we wish to know, then the number of turns of the roller will 

 tell us this amount ; or to go back to our water analogy, if we had the 

 weighted board to show current strength, and had not the paddle-wheel 

 to show total quantity, we might use the board to incline a disk in con- 

 tact with a roller, and then drag the roller steadily along by clock-work. 

 The number of turns of the roller would give the quantity of water. 

 Instruments that will thus add up continuously indications at a time, 

 and so find amounts during a time, are called integrators. 



The most important application that I have made at present of 

 the integrator described is what I have called an engine-power meter. 

 The instrument is on the table, but, as it is far too 

 small to be seen at a distance, I have arranged a large 

 model to illustrate its action. The object of this ma- 

 chine is to measure how much work an engine has 

 done during any time, and show the result on a dial, 

 so that a workman may read it off at once without 

 having to make any calculations. 



Before I can explain how work is measured, per- 

 haps I had better say a few words about the mean- 

 ing of the word " work." Work is done when press- 

 ure overcomes resistance, producing motion. Neither 

 motion nor pressure alone is work. The two factors, pressure and mo- 

 tion, must occur together. The work done is found by multiplying 

 the pressure by the distance moved. In an engine, steam pushes the 



Fig. 4. 



