Page:Philosophical Transactions - Volume 001.djvu/312

 And whereas 'tis observed, that upon the main Sea-shore the Current of the Ebbings and Flowings is sometimes swifter, and sometimes slacker, than at others, so as in the beginning of the Floud the Tide moves faster but in a small degree, increasing its swiftness constantly till towards the Middle of the Floud, and then decreasing in velocity again from the Middle till to the top of the High-water, it is supposed, that in Equal spaces of Time, the Increase and Decrease of velocity, and consequently the degrees of the Risings and Fallings of the same, in Equal spaces of time, are performed according to the Proportion of Sines.

But 'tis withall conceived, that the said Proportion cannot hold exactly and precisely, in regard of the Inequalities, that fallout in the Periods of the Tides, which are commonly observed and believed to follow certain Positions of the Moon in regard of the Equinox, which are known not to keep a precise and constant Course: so that, there not intervening equal portions of Time between one New Moon and another, the Moons return to the same Meridian, cannot be alwaies perform'd in the same Time, and consequently there must be at like Variation of the Tides in the Velocity, and in the Risings and Fallings of the Tides, as to equal spaces of time. And the Tides from New-moon to New-moon being not alwaies the same in number, as sometimes but 57, sometimes 58, and sometimes 59, (without any certain order of succession) is another evidence of the difficulty of reducing this to any great exactness. Yet, because 'tis worth while, to learn as much of it, as may be, the Proposer and many others do desire, That Observations be constantly made of all these Particulars for some Months, and, if it may be, years together. And because such Observations will be the more easily and exactly made, where the Tides rise highest, it is presumed, that a fit Apparatus being made for the purpose, they may be made about Bristol or Cheap-stow, best of any places in England, because the Tides are said thereabout to rise to ten or twelve fathoms; as upon the coast of Britanny in France, they do to thirteen and fourteen.

In order to which, this following Apparatus is proposed to be made use of. In some convenient place upon a Wall, Rock, or Bridge, &c. let there be an Observatory standing, as neer as may be to the brink of the Sea, or upon some wall; and if it cannot be well placed full where the Low water is, there may be a Channel cut from the Low water to the bottom of the Wall, Rock, &c: The Observatory is to be raised above the High-water 18. or 20. foot; and a Pump, of any reasonable dimension, placed perpendicularly by the Wall, reaching above the High water as high as conveniently may be. Upon the top of the Pump a Pulley is to be fastned, for letting down into the Pump a piece of floating wood, which, as the water comes in, may rise and fall with it, And because the rising and falling of the water amounts to 60. or 70. foot, the Counterpoise of the weight, that goes into the Pump, is to hang upon as many Pulleys, as may serve to make it rise & fall within the space, by which the height of the Pump exceeds the height of the Water. And because by Rh