6 TIDAL EVOLUTION. 



But all phenomena are not of this kind. There are some 

 astronomical phenomena which, at least in their broadest and 

 most philosophical sense, differ entirely from this class. They 

 do not increase and then decrease in regular rythmic varia- 

 bility, and for this reason differ entirely from the more obvious 

 ones such as I have enumerated. To one of these I am about 

 to invite your attention for a while this evening, and I hope 

 to be able to show you that in the subject we are to consider 

 we are really dealing with one of the most important factors 

 in the evolution of our earth-moon system. 



All intelligent people who have lived by the seashore are 

 familiar with the rise and fall of the sea which takes place 

 twice in every diurnal rotation of the earth on its axis. Long 

 ages ago these tides ceased to be a mystery to man, and in 

 fact before their theoretical explanation was mathematically 

 explained man was perfectly acquainted with their action, 

 their intensity, their maxima and minima, known as spring 

 and neap, and most important of all, with the wonder- 

 ful fact that in some mysterious way these tides were con- 

 nected with the position of the moon in relation to the sun. 

 Both Pliney and Aristotle refer to the alliance between the 

 times of high water and the stage of the moon. 



Not however, until Sir Isaac Newton in 1687 presented 

 to the Royal Society his immortal Principia, was it possible 

 for men to understand the abstract principle of the cause of 

 the tides. In this celebrated work Newton, in a manner which 

 makes it seem as if he was divinely inspired, unfolded to the 

 astonished world, the mechanical theory of nniversal, grav- 

 itation, showing that every particle of matter in the universe 

 attracts every other particle with a force which is ever pro- 

 portionate to the co-attracting masses, and inversely propor- 

 tionate to the square of the distances separating them. 



The moon, therefore, attracts every particle of matter on 

 our earth, both solid and liquid. The solids, however, are 

 not enabled to yield in such a marked way as the liquids are; 

 for the latter are forced by the moon's attraction to rise bodily 

 toward her to a certain extent, and in this way cause the 

 phenomenon of high tide. It must not be forgotten that the 



