would have signified that it was high water at the Nore later than at London Bridge, and you would have had to add the times instead of subtracting one from the other.
When the tidal constant of a place is not given in the almanac, it can be found, provided one knows the tidal establishment of the place and also that of some standard port, e.g. London Bridge; for the difference between these two times is the tidal constant of the place, additive if the tidal establishment of the place is greater than that of London, subtractive if it be less. Thus if the chart gives the tidal establishment of the place as IV h., the difference between IV h. and 1 h. 58 m. (which is the tidal establishment of London Bridge) gives the tidal constant + 2 h. 2 m., which can be applied to the London Bridge tide tables, in the manner before explained, in order to find the hour of high water for the day.
It must be borne in mind that strong winds often accelerate or retard high water and affect the range of the tide. Thus on the Thames, with a gale from the East, high water will be earlier and the rise of the tide will be higher than would be the case if the conditions were normal.
When shaping one's course at sea and establishing one's position in the manner that will be explained in the next chapter, it is often of great importance to know at what hour the tide turns and at what speed it runs at different periods of the
