Lesson xxxi.] TIDES. 211 



meridian of the place has passed under the lumi- 

 nary. 



The force of the sun to raise the Tides, is about 

 a third that of the moon : but it is evident, that 

 when the forces of both conspire,"so as to elevate 

 and depress the water in the same places, then the 

 Tides are greatest, and are called Spring Tides, 

 which happen about the new and full moon. 



When the moon is in her quarters, she elevates 

 the water most where it is most depressed by the 

 sun, and the contrary ; and the Tides, being 

 raised by the difference of their forces, are least, 

 or Neap Tides. But because of the continuation 

 of motion, these effects are greatest and least, 

 some time after the forces are : so .that the 

 greatest Spring Tides commonly happen three 

 days after the new and full moons; and the least 

 Neap Tides three days after the first and third 

 quarters. 



By the action of the sun the time of high water 

 is also changed, being sometimes sooner, and 

 sometimes later than it would happen by the action 

 of the moon alone. For in the transit of the moon 

 from new or full to a quarter, when the tide raised 

 by the sun alone would precede that raised by 

 the moon alone, the high water which is produced 

 by their united actions, will happen at an interme- 

 diate time, nearer to, but yet before, the time at 

 which it would be raised by the action of the moon. 

 But when the moon is passing from her quarters to 

 the new or full moon, since the sun alone would 



produce 



