226 POPULAR LECTURES AND ADDRESSES. 



still supposed to extend over only a small portion 

 of the earth's curvature, and all the six lakes 

 communicate with one another freely by canals or 

 underground tunnels : there will be no sensible tides 

 in the lakes A and A' ; in B and B' there will be 

 high water of if feet above mean level when the 

 moon or anti-moon is in the zenith, and low water 

 of if feet below mean when the moon is rising 

 or setting ; and at C and C' there will be tides 

 rising and falling 7/8 of a foot above and below the 

 mean, the time of low water being when the moon 

 or anti-moon is in the meridian of A, and of high 

 water when they are on the horizon of A. The 

 simplest way of viewing the case for the extreme 

 circumstances we have now supposed is, first, to 

 consider the spheroidal surface that would bound 

 the water at any moment if there were no dry land, 

 and then to imagine this whole surface lowered or 

 elevated all round by the amount required to keep 

 the height at A and A' invariable. Or, if there be 

 a large lake A in any part of the earth, communi- 

 cating by canals with small lakes over various parts 

 of the surface, having in all but a small area of 

 water in comparison with that of A, the tides in any 

 of these will be found by drawing a spheroidal 

 surface of if feet difference between greatest and 

 least radius, and, without disturbing its centre, 

 adding or subtracting from each radius such a 

 length, the same for all, as shall do away with rise 

 or fall at A. 



