332 OF TIDES* 



effect is the same as at the change, and in both cases occasions what 

 we call spring tides : but at the quarters, the sun's action dirniiiisjie.* 

 the edect of the 1110011*3 action, so that they rise a little under and 

 opposite the sun, and fall as much under and opposite the moon, 

 these two luminaries acting obliquely on each other, and producing 

 what is called neap tides. 



The spring tides, however, do not happen precisely at the full and 

 change of the moon, nor the neap tides at the quarters, but about 

 two days later. In this, as in many other cases, the effects are not 

 greatest, or least, when the immediate influence of the cause is 

 greatest or least : as, for instance, the greatest heat of summer is 

 wot at the time of the solstice, but some weeks after; and if the 

 actions of the suh and moon should be suddenly suspended, the 

 tides would continue for some time in their usual course. The v;t- 

 riations of the moon's distance from the earth produce a sensible 

 difference in the tides. When the moon approaches the earth, her 

 action on every part increases, and the differences of her action in- 

 crease in a higher proportion as the moon's distances decrease. Ac- 

 cording to Sir Isaac Newton, the tides increase as the cubes of the 

 distances decrease ; so that the moon, at halt' her distance, would 

 produce tides eight times as great. The sun being nearer the earth 

 in winter than in summer, the spring tides are highest, and the neap 

 tides lowesf, about the time of the equinoxes, a little after the 

 autumnal and before the vernal ; and, on the contrary, the spring 

 tides lowest and the neap tides highest at the solstices, when the 

 sun is most distant from tiie equator. When the moon is in the 

 equator, the tides are equally high in both parts of the lunar day ; 

 butas the moon declines towards either pole, the tides are alternately 

 higher and lower at places having north or south latitude : while 

 the sun is in the northern signs, the greater of the two diurnal 

 tides in our climates is that arising from the moon above the hori- 

 zon : when the sun is in the southern signs, the greatest is that aris- 

 ing from the moon below the horizon. Thus the evening tides in 

 summer are observed to exceed the morning tides, and in winter the 

 morning tides exceed the evening tides : the difference at Bristol is 

 found to be fifteen inches, at Plymouth twelve. It would be still 

 greater, but that a fluid always retains an impressed motion for 

 gome time, and consequently the preceding tides always affect 

 that follow. 



