PRESIDENT’S ADDRESS. 51 
the universe to the tides, and his theories had considerable 
vogue. A quotation from Bernhardus Varenius, who wrote 
his Geographia Generalis in 1650, well shows the condition 
of men’s minds with regard to this subject in his time. 
Varenius writes :— 
“To explaim the Cause of the Swelling and Swaging of the 
Sea, vulgarly called its Flux and Reflux: 
‘“There is no Phenomenon in Nature that has so much 
exercised and puzzled the Wits of Philosophers and learned 
men as this. Some have thought the Earth and Sea to be 
a living Creature, which, by its Respiration, causeth this 
ebbing and flowing. Others imagined that it proceeds, and 
is provoked, from a great Whiripool near Norway, which for 
six Hours absorbs the Water, and afterwards disgorges it 
in the same space of Time. Scaliger, and others, supposed 
that it is caused’ by the opposite Shores, especially of 
America, whereby the general Motion of the Sea is ob- 
structed and reverberated. But most Philosophers, who 
have observed the Harmony that these Tides have with the 
Moon, have given their opinion that they are entirely owmg 
to the Influence of that Luminary. But the Question is, 
what is this Influence? To which they only answer that 
it is an occult Quality, or Sympathy, whereby the Moon 
attracts all moist Bodies. But these are only Words, and 
signify no more than that the Moon does it by some means 
or other; but they do not know how: Which is the Thing 
we want.” 
Galileo and Wallis explained the tides as due to the dif- 
ferent speeds in space of different places on the 
earth’s surface, the side away from the sun moving 
eastward with a greater. velocity than the side 
‘towards the sun; but they could not clearly explain why 
there should be two high waters and two low waters daily 
instead of one. Galileo thus looked upon the tides as giving 
us evidence of the rotation of the earth. 
Our modern tidal theory is founded essentially upon the 
work of Newton, who first clearly explained the nature of 
the tide-generating force in his Princ7a in i687. He 
showed that the difference of the forces which the moon 
exerts upon a particle at the earth’s surface and upon a par- 
ticle at the earth’s centre constitutes a force which tends to 
move the particle. at the surface relatively to the earth’s 
centre. This difference constitutes the Tide-Generating 
Force. At any particular spot this can be resolved into 
horizontat and vertical components. The vertical com- 
ponent simply has the effect of slightly altering the apparent 
