We have already explained that the time of high water at any particular spot at the full and change of the moon is indicated on the chart—thus, for instance, H.W. at F. and C. 6 h. 10 m., which signifies high water at full and change of the moon at six hours ten minutes. If we have no tide tables at hand, we can roughly calculate the time of high water for the day by adding forty-eight minutes for every day that has passed since the last full or new moon to the time at full and change given on the chart.
Fig. 53
But a more accurate method is to refer to the Admiralty tide tables, or, what will answer the purpose equally well, to Pearson’s Nautical Almanac, a little book which we strongly recommend to the yachtsman. Here he will find daily tide tables, morning and afternoon, for London and other principal English ports, together with the height of the rise in feet.
Besides these, there is an extensive lists of ports and positions on the coast of England and Europe with their Tidal Constants. The constant for a given place is the number of hours and minutes that are to be added to or subtracted from the time of high water at the standard port or port of reference in order to obtain the time of high water at the given place.
For instance, supposing London to be the standard port, as it is in Pearson’s Almanac, and we require to know the hour of high water at Portland Breakwater on a given day. We first refer to the table of constants, and find + 5 h. 3 m. to be the constant of Portland Breakwater. We then turn to the London tide table, and find the time of high water for the day—morning or afternoon, as the case may be. We add five hours and three minutes to this, and the result will be the required time. Had the sign before the constant been-instead of + we should have subtracted and not added.
If we require the high water at a port where the tidal constant is not given in the tables, but where high water at full and change is given on the chart, find the high water at full and change of some port—London, for example, whose constant is in the tables. Subtract the lesser of these two times from the greater, the remainder will be the constant of our port—additive if the full and change at the port be greater than that of London, subtractive if it be less.
CHAPTER XIV.
WEATHER WISDOM.
Use of the instruments—Forecasting weather from natural phenomena, etc.