110 FERGUSON'S LECTURES. 



LECT propels Mr. Symington's steam-boat has only six float-boards in 

 ^^/^. its whole circumference. 



Since the water escaping from an under-shot wheel still re- 

 tains a part of its velocity, it is obvious that this may be em- 

 ployed for turning a second wheel, if it be desirable to preserve 

 as much as possible of the force. In this case, by causing the 

 tirst wheel to move with two thirds of the velocity of the stream, 

 the whole effect of both will be one third greater than that of a 

 single wheel placed in the same stream ; but it must be consi- 

 dered that the expense of the machinery will also be materially 

 increased. Vide DR. YOUNG. 



The following comparative view of the effects of wind, water, 

 and animal force, when applied as prime movers in machinery, 

 is extracted from the Editor's treatise on Steam Engines. 



From the most accurate observations, it appears, that the 

 physical powers of the human race differ very widely, not only 

 in various individuals, but also in different climates ; the value of 

 a man, therefore, as a working machine, will not be so great 

 beneath the torrid zone as in the more temperate climate of 

 Europe. This will serve to illustrate the great advantage which 

 our Colonists, particularly in the West Indies, would derive 

 from the more general employment of inanimate force ; the day- 

 labour of a negro in the sugar countries amounting to little more 

 than one-third of that performed by a European mechanic. 



A labourer, working ten hours a day, can raise in one mi- 

 nute a weight equivalent to 3750 pounds one foot high, or about 

 sixty cubic feet of water in the same time : while the power of a 

 horse, working eight hours per day, may be correctly averaged 

 at 20,000 pounds. Smeaton states, that this animal, by means of 

 pumps, can raise two hundred and fifty hogsheads of water ten 

 feet high in an hour. It is a well-known fact, also, that men, 

 when trained to running, are able, on the average of several days 

 being taken, to outstrip the fleetest horse: and yet it will be 

 seen from the above statement, that his force, if properly applied, 

 is at least six times that of the most powerful man. 



The use of water, as an impelling power, both for the turn- 

 ing of machinery and other purposes connected with the useful 

 arts, appears to have been known at a very early period. Vitru- 

 vius describes a variety of machines for this purpose, the earliest 

 of which were employed merely to raise a portion of the fluid by 

 which they were impelled. The most simple method of apply- 

 ing this element as a mechanical agent, evidently consisted in the 

 construction of a wheel, the periphery of which was composed of 



