(656)
A Letter written to Dr. John Wallis by Mr. Henry Philips, containing his observations about the True Time of the Tides.
WOrthy Sir, Being desired by Mr. O. to give in, what informations I could, concerning the Tides, I have made bold to present this Paper to your Consideration; which though it have little or no relation to your more curious Philosophical Experiments, yet, I hope, will be of very good use for the finding out the True time of the Tides at all times of the Moone, which is (I conceive) of as great concernment, as any thing in the Motion of the Tides.
For, this time of the Tides, though it be a very necessary thing to be known, yet is very rudely and slightly reckoned up by most Seamen and Astronomers; most of them reckoning, as if the Moone being upon such a set point of the Compasse (as the Seaman calls it) or so many houres past the Meridian (as the Almanack-Makers reckon) it were High-Tide in such and such a Port at all times of the Moone. And thus they reckon the Tides every day to differ constantly 48 m. As for instance; A South-West Moone makes a full Tide at London, that must be understood, that it is High-Tide at London when the Moon is three hours past the Meridian. Now this is true indeed at the New and Full Moon, but not at other times of the Moone, which few take any notice of: only Mr. Booker had wont to give this Caveat, that about the first and last quarters of the Moone, the Neap-tides did not flow so long as the Spring-tides by one point of the Compasse; but he gives no rule to proportion the difference.
But observing this more narrowly, I find, that at London the Tides fall out at the least two points, that is, one hour and an half sooner, in the Quarters then in the New and Full Moone. Now this being a very considerable difference of time, which might very well make many Seamen and Passengers to lose their Tides, I set my self to watch this difference of the time of the Tides, and to find out some Rule, how to proportion the time of the Tides between the Spring-tides and the Neap-tides, and I found by many trialls, that the true time of the Tides might be found
