12 



Prime Movers. 



foot high, per minute ; and this is the standard which has "been 

 adopted in estimating the power of steam-engines. It is equiva- 

 lent to 15,840,000 lbs. raised one foot high in a working day of 

 eight hours. According to Gerstner, the following represents 

 the work done by the different animals : — 



Animals. 



Pounds 

 Weight. 



Mean 

 Effort in 

 Pounds. 



Mean 



Speed per 



Second. 



Hours 

 per 

 Day. 



Pounds 

 per 



Second. 



Effect 

 per 

 Day. 



Man 



150 

 600 

 600 

 500 

 360 



30 



120 

 120 



100 



72 



2-5 

 40 

 25 

 35 

 2-5 



8 

 8 

 8 

 8 

 8 



75 

 480 

 180 

 350 

 180 



2,160,000 

 13,824,000 



8,640,000 

 10,080,000 



5,184,000 



Draught-horse ... 

 Ox 



Mulo 







A horse, on the authority of Desaguliers and Smeaton, is 

 usually considered as equivalent to five men. Bossut reckoned 

 an ass as equivalent to two men. 



The power of animals is derived from the combustion which 

 is carried on in their bodies ; and the heat derived from this 

 source was originally absorbed from the sun's rays during the 

 growth of those plants which, mediately or immediately, form 

 their food. And hence, in reality, their force is derived from 

 the sun. 



Having availed himself of the strength of animals, man was 

 a long time before he perceived that he might obtain a motive 

 power from water. Hand-mills, termed querns, were used for 

 grinding corn long after the still ruder method of pounding it 

 had been wholly, or in part, discontinued ; subsequently these 

 were fitted with shafts, and cattle were attached to them ; but, 

 as Strabo informs us, water-mills were not introduced at Home 

 until about seventy years before the Christian era. It has been 

 estimated that a ton and a-half of water per minute, falling one 

 foot, will grind and dress a bushel of wheat per hour. The 

 cost of putting up any hydraulic machine is nearly the same as 

 that of a steam-engine of the same power ; but the force derived 

 from it is less expensive. — The power obtained from rivers and 

 streams, also, is derived from the sun. For the water is raised, 

 during evaporation, by the sun's rays ; and produces its effect, 

 while falling back to its original position. 



A vast force is generated by the rising and falling of the 

 water which constitutes the tide ; and it has, in a few instances, 

 been turned to a practical use, by means of tide-mills. — The 

 motion obtained in this way, however, forms an exception, not 

 being derived from heat, but from the attractive action of the 

 sun and moon. 



