zz 3 -Sir ISAAC N E W T O NTs Book IV.

from us is about 30 diameteri of the earth. Thus the in-equality of the action of the earth on the parts of a drop ofwater is altogether insensible, because the diameter os the dropis an insensible quantity compared with its distance from thecentre of the earth.

However, the immense bulk of the fun makes the effectstill sensible at so vast a distance ; and therefore, tho’ theaction of the moon has the greatest share in producing thetides, the action of the fun adds sensibly to it when they con-spire together, as in the change and full of the moon, whenthey are nearly in the same line with the centre of the earth,and therefore unite their forces; so that then the tides aregreatest-, and are what we call the spring tides. The actionof the fun diminishes^ the effect of the moon’s action in thequarters, because the one raises the water in that cafe wherethe other depreffes it ; and therefore the tides then are least ;and these we call the neap tides. Tho’, to Ipeak accurately,the spring and neap tides must be some time after ; because,as in other cafes, so in this, the effect is not greatest or leastwhen the immediate influence of the cause is greatest .or least.As the greatest heat, for example, is not on the solstitial day,when the immediate action of the fun is greatest, but sometime after.

That this may be more clearly understood, let it be con-sidered, that, tho’ the actions of the fun and moon were tocease this moment, yet the tides would continue to have theircourse for some time : For the water where it is now highestwould subside and flow down on the parts that are lower, till,by the motion of descent, being there accumulated to too greata height, it would necessarily return again to its first place,tho’ in a less measure, being retarded by the resistance arising