626 PRESIDENTS ADDRESS. 



think of the aions of millions of years which have elapsed since 

 the moon began at the average rate of a small fraction of a 

 second in a thousand years to increase the length of the terres- 

 trial day by some 21 hours! And this action must go on until 

 our day becomes longer and longer, until it stretches out to a 

 month, and the moon will then face the earth continuously at one 

 spot, and lunar tides will cease. The moon and the earth will then 

 be each revolving around its axis in equal times, and so must 

 face one another in one position. The tidal influence of the sun 

 will still go on, and the ultimate result will be that in the 

 infinitely remote future the period of the earth's revolution will 

 stretch out to a 3^ear, and the earth will then face the sun as the 

 moon now faces the earth. It will then be perpetual day on the 

 side of the earth which is turned to the sun, and perpetual night 

 on that which is turned away. That is assuming that the sun is 

 still capable of giving off heat and light. And all this will have 

 been due to the tides ; so we see how profound are the results 

 induced by the physical properties of the ocean. The fact of the 

 absence of tides in enclosed seas of even considerable size, such, 

 for example, as the Caspian and Black Seas, and the very insig- 

 nificant tides experienced in the Mediterranean, are due to the area 

 of these not being sufficiently great for the moon to be able to 

 raise a tide; the moon pulls such areas of water practically as a 

 whole. 



Considering the enormous effect of the moon when acting on a 

 large surface area, it is not a little surprising, at first sight, to 

 find that appliances of the utmost delicacy are required to 

 measure, or even to demonstrate, our satellite's attraction. The 

 amount of this attraction can be calculated readily enough, but 

 to show its existence experimentally is a very difficult matter. 

 This was done by Professor G. H. Darwin at Cambridge, by 

 suspending, with most elaborate precautions, a mass of copper 

 weighing several pounds, on a long wire, in such a position that 

 when the moon was at right angles to the weight, and therefore 

 pulling horizontally, the minute displacement could be measured 

 by means of a tiny mirror placed on a wire torsion frame attached 

 to the weight and reflecting a beam of light on to a scale. 



