{?i the Rise and Fall of the tides. 253 



foot. Supposing, therefore, we were in possession of an unlimited 

 number of 7 7 2 -pound weights, and were to employ in the most 

 judicious manner the force thus evolved in their downward pro- 

 gress, we should have an unlimited reservoir of power which 

 could be converted at will into light, heat, electricity, or chemi- 

 cal affinity, and could be made to toil for human benefit without 

 any corresponding expenditure of human labour so long as the 

 weights continued their downward progress unarrested. If, how- 

 ever, any good were to be gained by such a machine, it must be 

 managed so that the motive force — gravitation — should always 

 remain on the pull, and this is, and always will be, the obstacle 

 to the attainment of perpetual motion ; the act of overcoming the 

 force of gravity to re-raise the weights, requiring the expenditure 

 of exactly the same amount of power as has been generated dur- 

 ing their downward fall ; and so, before we can seriously discuss 

 the feasibility of such a machine, we must find a perpetual flow 

 of gravitating force always at hand, craving to be satisfied, and 

 yet inexhaustible. In other words, we must construct a clock 

 which will wind itself up when the weight has run down, without 

 any expenditure of human power. 



Sitting by the sea-shore a few days since, we could not help 

 noticing the large reservoir of mechanical power existing in the 

 ocean. We do not refer to the noisy dash of the waves as they 

 break upon the beach, but to the infinitely mightier, although 

 silent and progressive, energy exerted in the gradual rise and 

 fall of the tides. Compared with the stupendous power capable 

 of being utilised for man's benefit, and present in the rise or fall 

 of millions upon millions of tons of water through a space of ten 

 or twenty feet four times a-day, all the steam, water, or wind 

 power in the world, together with the united muscular force of 

 every living being, human and animal, sink into utter insignifi- 

 cance. We will try to form some idea of this power. Let us 

 suppose that by the action of the tides the difi"erence of level of 

 the surface of the ocean at a certain spot, is 21 feet between high 

 and low water ; omitting for the present all consideration of the 

 power of the subjacent liquid, what is the mechanical value of a 

 space of 100 yards square of this water ? 100 yards square by 

 21 feet deep equals 70,000 cubic yards of water, which are lifted 

 to a height of 21 feet, or to 1,470,000 cubic yards Hfted to a 

 height of 1 foot. Now, since one cubic yard of water weighs 

 about 1683 pounds, 1,470,000 cubic yards weighs 2,474,010,000 



