244 Introduction to the Study of Science 



About the year 1780, James Watt, who had manufactured 

 and was then trying to sell steam engines to mine operators, 

 discovered that he had to compete with the horse as a motive 

 power. Coal and water were hoisted from the mine by a rope 

 running over a pulley or wheel at the top of the shaft, and to one 

 end of the rope the horse was hitched. As the horse walked 

 away and pulled, it lifted the load. Watt quickly found it 

 necessary to prove that his engine could do more work in the 

 same period of time than the horse. To do this he had first to 

 show what a horse power represents in foot-pounds per minute or 

 other unit of time. By careful experiments he found that an 

 average draft horse could lift a load of 150 pounds when walking 

 220 feet a minute ; or it could lift a load of 75 pounds when walk- 

 ing at the rate of 440 feet a minute, or 300 pounds when walking 

 110 feet a minute. In all these cases the horse is doing the 

 same amount of work. The product of any set of two terms 

 (weight and distance) given by each trial is 33,000, the foot- 

 pounds of work in one minute. This was therefore taken by 

 Watt, and since then generally used, as representing one 

 horse power, that is, the performance of work at the rate of 

 33,000 foot-pounds a minute. 



It is a simple matter now to compute the horse power of the 

 given fall of water. The fall, as you see, performs in one minute 

 187,500 foot-pounds of work. The horse power is to be found 

 from the proportion 1 (h.p.) : x (h.p.) : : 33,000 : 187,500, or x 

 equals 5.6 horse power. That is, the horse power of a given 

 fall of water is equal to the number of times 33,000 is contained 

 in the number of foot-pounds of energy produced by the water 

 per minute. 



Exercise. 1. Suppose that 650,000 tons of water pass over the 

 Niagara Falls each minute, and the useful head is 160 feet. What 

 will be the total horse power of the water? 



2. The Keokuk power plant delivers 300,000 horse power of elec- 

 trical energy, which is only 85 per cent of the total energy furnished 

 by the water. What is the total horse power of the water? 



