PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 581 



is, however, too small to make itself felt ■within the period over which 

 observations of the subject extend." 



Useful Application of Tidal Power. 



The tide is used to register its own movements. The self-registering 

 tide-gauge, used in the United States coast survey, was constructed by 

 the well known American inventor, Joseph Saxton. A full description of 

 this instnunent may be found in the Coast Survey Report for 1853. The 

 importance of this instrument in recording an unusual action of the ocean 

 will be appreciated by citing a single case. On the 23d and 25th of De- 

 cember, 1854, the tide-gauge at San Diego, registered unusual curves, 

 which were noticed by Lieut. Trowbridge, of the coast survey, and in com- 

 municating this fact to Prof. Bache, he added, "there is every reason to 

 presume that the effect was caused by a submarine earthquake." No shock 

 had been felt along the Pacific coast. On the 20th of June following, ac- 

 counts were received in this city of a violent earthquake at Japan, by 

 which the Russian frigate Diana, in the port of Simoda, was so injured as 

 to be rendered useless. 



Letters from Japan gave the time at which these shocks occurred; one 

 of these claimed to be an extract from the log-book of the Diana. From 

 these statements *and the curves registered both at San Diego and San 

 Francisco, Prof. Bache calculated the rate of motion of the earthquake 

 waves were at from 355 to 370 miles per hour, and the length of the waves 

 ^ on the San Diego path, at from 182 to 1-92 miles; that on the San Francisco 

 path, at 210 to 217 miles. The average depth of the ocean on the San 

 Francisco path was also calculated, from the length and rapidity of the 

 wave, to be 2,500 fathoms; that on the San Diego path at 2,100 fathoms. 



The waves thus measured by Prof. Bache, produced by the heaving of 

 the earth, form a third class which may be distinguished as v/aves of dis- 

 placement. 



The most important application of the tidal power is for the movement 

 of various kinds of machinery used in the arts. Mills thus driven derive 

 their power indirectly from the moon, while all other water-mills depend 

 on the fall of water which has been elevated by the sun in the process of 

 evaporation. 



Water, as used at natural and artificial waterfalls for driving mill-wheels, 

 is, without doubt, our cheapest motor. The tide mill is not very efiicient 

 because its power is never great and cannot act continuously. In conclu- 

 sion, the Chairman invited those present to give brief descriptions of tide 

 mills that had come under their observation. The only one, now in his 

 memory, was at Stamford, Conn., on Long Island sound. It was originally 

 used as a flouring mill, but afterwards for grinding drugs and condiments. 



Mr. Dibben said there were, to his knowledge, several tide mills on Long 

 Island, and one on Staten Island. Many present may remember tlie old 

 Gowanus bay, now filled up and forming part of the city of Brooklyn. As 

 late as 1846, this was the pond of a tide mill belonging to one of our promi- 

 nent citizens and used for grinding dye stuffs. There are several kinds of 

 wheels used in tide mills — that called a current wheel is used in some of 

 the rapid streams at the west. 



