THE TIDES. 273 



Withont entering into details, which would x^equire mathematical demon- 

 strations, we shall remark only, that the proximity of the sun and moon 

 seems to be the cause to which we must refer the extraordinary equinoctial 

 tides, which happen most frequently, the one before the vernal equinox, 

 and the other after the autumnal ; that is, both of them at the time when 

 the sun, passing through the meridional signs, is nearest us. But this does 

 not happen every year, because there are sometimes variations produced 

 by the situation of the orbit of the moon, and by the distance of the 

 syzigies from the equinoxes. 



(222.) This, then, is the general Theory of the Tides, and from these 

 observations their general laws may be inferred ; but it has been reserved 

 for later times to pursue the inquiry into detail, and to develop the minor 

 effects which modify, and in some places totally change, the character of 

 the Tides. It is chiefly to the late Bev. Dr. Whetvell, Master of Trinity 

 College, Cambridge, and to the late Sir John Luhhock, that our present 

 knowledge of the Tide laws is owing, and from their observations we will 

 give some extracts. 



(223.) In the Rev. Dr. Whewell's papers on the subject of the Tides, he 

 commences: — "Ever since the time of Newton, his explanation of the 

 general phenomena of the Tides, by means of the action of the moon and 

 the sun, has been assented to by all philosophers who have given their 

 attention to the subject. But, even up to the present day, this general 

 explanation has not been pursued into its results in detail, so as to show 

 its bearing on the special phenomena of particular places — to connect the 

 actual Tides of all the different parts of the world — and to account for their 

 seeming anomalies. With regard to this alone, of all the consequences of 

 the law of universal gravitation, the task of bringing the developed theory 

 into comparison with multiplied and extensive observation is still incom- 

 plete ; we might say, is still to be begun.* 



(224.) The Tidal Wave. — The tidal wave is not owing to the transfer of 

 the body of water, which would be a current, but to an elevation of its 

 surface. This motion is, as may be readily conceived, compatible with 

 immense velocity ; and it may be taken as a rule, that the broader the 

 wave, the greater will be its velocity. If the earth were in equilibrium, 

 and its surface entirely covered with water, and under the influence of the 

 moon's attraction, it would assume the form of an ellipsoid, having the 

 semi-axis directed towards the moon longer by about 58 inches than that 

 transverse to it ; that is, the water would become higher by that amount. 

 This is merely adduced to show what may be the amount of the luni-tidal 

 wave, without entering into any other considerations. 



(225.) Velocity of the Tidal Wave. — As the whole of the tidal wave must 

 circulate around the globe in twenty-four hours nearly, the velocity must 

 be very great ; but it is greatly modified. In the middle of the i^.tlantic 

 \t would appear to travel at the rate of about 700 miles an hour, but on 



* " Essay towards a First Approximation to a Map of Cotidal Lines ;" PhilosophioR' 

 Transactions of the Royal Society, 1833, page 147. 



2^. A. 0. 36 



