290 NEWTON S PRINCIPIA. 



greatest in latitude a ; thus the height of the tide varies 

 with the latitude. 



Seventhly. But the difference between the latitude of the 

 place and one vertex is not the same as that between it 

 and the other vertex. Hence, though we have two tides 

 every day, these two tides will be of unequal magnitude. 

 The moon s orbit is inclined at but a small angle to the 

 ecliptic; hence, speaking generally, her orbit will have 

 nearly the same position relative to the equator that the 

 sun s orbit has. A line drawn through the centre of the 

 earth and moon is the axis of the tidal spheroid. The tides 

 which occur on the side next the sun, when the sun has 

 north declination, will be greater than the tide on the op 

 posite side of the earth. Therefore in summer the day tides 

 are greater than the night tides ; similarly in winter the 

 night tides are greater. &quot; If the pole of the tidal flood 

 follow the moon, say at six hours, the pole will be north 

 from the time the moon is six hours west of the sun to the 

 time when she is six hours east, that is, from the time when 

 the high tide is at noon to the time when it is at mid 

 night.&quot; In such a case therefore the afternoon tide is 

 greater than the morning tide in summer and less in 

 winter. Similar reasoning will apply when the &quot; age of 

 the tide &quot; is twelve, thirty, &c. hours. 



Eighthly. If the tide be brought to any place by two un 

 equal channels, the two tides following the two transits of the 

 luminary across the neighbouring oceans will meet one ano 

 ther at this place, and form a compound tide. Suppose one 

 tide to be delayed six hours, it will be in exact opposition to 

 the tide which arrives by the other path. If the two were 

 equal, the waters would stagnate, and there would be no 

 tide at this particular point. But if the luminary be not 

 in the equator, the two tides are unequal ; hence, compound 

 ing the two, there will be one tide every twenty-four hours. 



Newton quotes from Halley, as an example of this, the 



