PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 579 



their useful applications by inercbanism, was then taken up. The follow- 

 ing remarks were made by the Chairman : 



The profound Kepler was the first to explain, in a general way, the 

 cause of tides. By intuition, apparently unconnected with his deduction from 

 his third grand celestial law, he proclaimed " that all bodies attract each 

 other; the waters of the ocean are held in their bed by the earth, but are 

 elevated under the moon and on the opposite side of the earth because the 

 earth is less attracted by the moon than nearer waters, but more than the 

 waters on the opposite side." The mathematical demonstration that the 

 power of the moon and sun, in this respect, is in accordance with the law 

 of gravitation, was the work of Newton; but his equilibrium theory could 

 not be satisfactorily applied to all the phenomena observed. It was left 

 for Laplace, in his treatise on Celestial Mechanics, to explain definitely the 

 many apparently discordant movements of the sea by what is known as the 

 dynamical theory of tides, in which the laws of hydrodynamics have an 

 important application. The cubic measure of the ocean is estimated at T88 

 millions of cubic miles, and taking the density as found two miles below 

 the surface as the mean, its weight is about l-HSSth of that of the whole 

 globe. Although the sun is 355,000 times heavier than the earth, yet at 

 the distance of 90,000,000 of miles, its influence upon the ocean is far less 

 than that of the body only l-15th of the weight of the earth acting at a 

 distance of 240,000 miles. The mass of the sun is more than 26,500,000 

 times greater than that of the moon,but being 396 times her distance from 

 the earth, the height of the tidal wave produced by it, compared with that 

 raised by the moon, is as 38 to 100. When these two attracting bodies 

 are in conjunction, or are on opposite sides of the earth, their joint action 

 produces the highest water, called the Spring tides. When the moon is in 

 its quadratures, that is 90° from the points of conjunction or opposition, its 

 lowest effect is at the point of the highest effect of the sun, and their joint 

 tide — the difference of the separate ones — is called the Neap tide. The 

 rise and fall of Spring and Neap tides are nearly as seven to three. The 

 action of either luminary consists in the difference of the attractive force 

 exerted by it on the solid core of the earth and its covering, the ocean, and 

 the effect is to form two waves on opposite sides of the earth. Were there 

 a continuous sea on the line of the Equator, the great tidal or primary 

 wave would move at the rat6 of about 1,050 statute miles per hour, but as 

 the sea is broken up into three great basins, and their only connection of 

 note being southward of the continent, the tidal movements are compli- 

 cated, and greatly diminished in velocity. In each basin the waves 

 must start anew from the eastern shore, but in the southern sea the cir- 

 cular course is unimpeded. It also spreads across the three great basins 

 diagonally, so as to overtake and commingle with the tidal wave proper. 

 These commingling waves, the regular currents of the ocean, its varyino* 

 depths, the shape of its shores and estuaries, are causes of perplexity in 

 tidal calculatians. Yet, through all, it may be perceived that there is a 

 tide corresponding in its semi-diurnal rise and fall with the lunar day of 

 twenty- four hours and fifty- four minutes. 



For a detailed description of the tides on our Atlantic coast, in the Gulf 



