A GLIMPSE THROUGH THE CORRIDORS OF TIME. 481 



flow which we call the tides. Even the children with their spades and 

 buckets know how the flowing tide will fill their moats dug in the sand 

 and inundate their mimic castles. In the ebb and flow of the tide we 

 have a mechanical engine of mighty power. I hope this evening to 

 point out the wonderful effect which tides have had on the earth in 

 times past, as well as the effect they will exercise in the future. It is 

 the tides which are to reveal to us a glimpse through the corridors of 

 time. 



The cause of the ebb and flow of the tide has long ceased to be a 

 mystery. In the earliest times it was noticed that the tides were con- 

 nected with the moon. Pliny and Aristotle both refer to the alliance 

 between the tides and the age of the moon. It is well known that the 

 tides on our coasts sometimes rise to an unusual height. Those who 

 dwell on low ground adjoining tidal rivers are painfully aware of this 

 fact by the floods which are often produced. Such occurrences gen- 

 erally take place at the time of new moon or of full moon. At first 

 quarter or last quarter the tides are even below the usual height. A 

 fisherman who has to regulate his movements by the tides will know 

 full well that at certain times the tides rise higher and fall lower than 

 at other times. He brings his boat out on the falling tide, he brings 

 it back on the rising tide, and, Avhen making the harbor after a night's 

 fishing, it would be natural to hear him say, " Oh, we shall run in easily 

 this morning, there is a strong tide, the moon was full last night." Or 

 if he had to cross a dangerous bank he would soon learn the difference 

 between the spring tide and the neap. Fishermen are not much ad- 

 dicted to abstract reasoning. For many centuries, perhaps indeed for 

 thousands of years, observant men might have known that the moon 

 and the tides were connected. But they did not know any reason why 

 this connection should exist. I dare say they did not even know 

 whether the moon was the cause of the tides or the tides the cause of 

 the moon. 



Nor is it easy to explain the tides. We were all taught that the 

 moon makes the tides. Yet I can imagine an objector to say, If the 

 moon makes the tides, why does it give Bristol a splendid tide of forty 

 feet, Avhile London is put off with only eighteen ? The true answer 

 is, that the height of the tide is largely affected by local circum- 

 stances, by the outline of the coasts, by estuaries and channels. It is 

 even affected to some extent by the wind. Into such details, however, 

 I do not now enter : all I require is, that you shall admit that the moon 

 causes the tides, and that the tides cause currents. In some few places 

 the currents caused by the tides are made to do useful work. A large 

 reservoir is filled by the rising tide, and as the water enters it turns a 

 water-wheel. On the ebbing tide the water flows out of the reservoir, 

 and again gives motion to a water-wheel. There is here a source of 

 power, but it is only in very exceptional circumstances that such a 

 contrivance can be worked economically. Sir W. Thomson, in his ad- 



VOL. XX. 31 



