tefpeB'mg the Caufe of the Tides* 137 



Verfd: when both luminaries are in the equator, and the 

 moon in perigeo, the tides rife higheft, particularly at oppo- 

 fition and conjunction : at the change, when the attractive 

 forces of the fun and moon are combined, the tide is raifed 

 to a greater height : at the full, when the moon raifes the 

 tide under and oppofite to her, the fun, afting in the fame 

 line, raifes the tides under and oppofite to him, whence their 

 conjunct effect is the fame as at the change, and in both cafes 

 occafions what we call fpring tides : but at the quarters, the 

 fun's action diminifhes the effect of the moon's action, fo 

 that they rife a little under and oppofite the fun, and fall as 

 much under and oppofite the moon, thefe two luminaries 

 acting obliquely on each other, and producing what is called 

 neap tides *. 



The fpring tides, however, do not happen precifely at the 

 full and change of the moon, nor the neap tides at the quar- 

 ters, but about two days later. In this, as in many other 

 cafes, the effects are not greateft, or leaf!:, when the imme- 

 diate influence of the caufe is greateft or leaft : as, for in- 

 fiance, the greateft heat of fummer is not at the time of the 

 folftice, but fome weeks after; and if the actions of the fun 

 and moon Ihould be fuddenly fufpended, the tides would con- 

 tinue for fome time in their ufual courfe. The variations of 

 the moon's difiance from the earth produce a fenfible dif- 

 ference in the tides. When the moon approaches the earth, 

 her action on every part increafes, and the differences of her 

 action increafe in a higher proportion as the moon's diftances 

 decreafe. According to Sir Ifaac Newton, the tides increafe 

 as the cubes of the diftances decreafe ; fo that the moon, at 

 half her diftance, would produce tides eight times as great. 

 The fun being nearer the earth in winter than in fummer, 

 the fpring tides are higheft, and the neap tides loweft, about 

 the time of the equinoxes, a little after the autumnal and 

 before the vernal ; and, on the contrary, the fpring tides 

 loweft and the neap tides higheft at the folftices, when the 



• In Pig. 2. Plate V. HZ ON rcprefents the earth; ABCD tlie 

 rroon's orbit. At the full and charge, the fun and moon act in the fame 

 line S P : at the quarters, the fun's influence in the line SOH counteracts 

 that < f the moon u£tiii£ in the dirceY'on MZN, an J produces neap tides. 



. Vol. VIII. T fun 



