TIDES
I have already explained how one can calculate the hour of high water at a place for any particular day, if the chart indicates the tidal establishment of that place—that is, the hour of high water there, at the full and change of the moon. But a more accurate method for finding the time of high water for any well-known port is to refer to a Nautical Almanac, such as Pearson's, which costs only sixpence, and is the most useful little book of its kind I know of. In this almanac are to be found tide tables for some of the principal ports in Great Britain, and also an extensive list of British and other ports and positions with their tidal constants—that is, the difference in time between the high water at these places and high water at some standard port, such as London Bridge.
Thus if you wish to ascertain the time of high water at the Nore for August 15, refer to the above-mentioned list and you will find that the tidal constant for the Nore is—1 h. 45 m., which signifies that it is high water there one hour and forty-five minutes earlier than at London Bridge. Then look up the London Bridge tide tables for the hour of high water on August 15, which we will suppose to be 3 h. 15 m. P.M. Deduct 1 h. 45 m. from 3 h. 15 m., and the result, 1 h. 30 m. P.M., will be the time of high water at the Nore for that day. If the sign before the constant had been + instead of - it
