LET. VIII. OF THE NATURE OF THE TIDES. I2Z -

we proceed; but, at present, it will be sufficientto observe that they always follow a certain gene-ral rule. For instance, if the tide be now athigh-water mark, in any port, or harbour, whichlies open to the ocean, it will presently subside,and flow regularly back, for about six hours,when it will be found at low-water-mark. Afterthis, it will again gradually advance for sixhours, and then return back, in the same time,to its former situation ; rising and falling alter-nately, twice a day, or in the space of abouttwenty-four hours.

The interval between its flux and reflux, is,however, not precisely six hours, but abouteleven minutes more; so that the time of highwater does not always happen at the same hour,but is about three quarters of an hour laterevery day, for thirty days, when it again recursas before. For example, if it be high water today at noon, it will be low water at eleven mi-nutes after six in the evening, and, consequently,after two changes more, the time of high waterthe next day, will be at about three quarters ofan hour after noon; the day following it willbe at about half an hour after one; the day afterthat at a quarter past two; and so on for thirtydays ; when it will again be found to be highwater at noon, the fame as on the day the ob-servation was first made. And this exactly an-swers to the motion of the moon; she rises

every