ARTICLE III.

FIRST DIVISION, SECTION 3.

TIDES.

BY THE REV. DR. WHEWELL.

Directions for Tide Observations.

1. In making tide observations, the main object is, in the first place, to refer the tides to the motions of the moon, by which they are, in most places, mainly governed.

For this purpose, the time and height of high water (and of low water) at each place must be obtained; and this time will have to be compared with the time of the moon's passage across the meridian of the place.

The latter time (the time of the moon's transit) may be known by the common table given in the Nautical Almanac, or in other books of the same kind.

2. The time of high water (and low water) may sometimes (when the sea is calm) be ascertained with sufficient accuracy by observing the surface of the sea, where it washes a vertical scale fixed in the open water, and divided into feet and inches. The moment when the water is highest (and lowest) must be observed by a watch or clock, well regulated or corrected for its error.

3. In general, the waves will make it difficult to observe the moment of the highest (and lowest) open water with much accuracy. The following methods may be used to make the observations more accurate :-An upright tube, open below and above, may be placed in the water, reaching above the high water and below the low water (or two tubes, one for high water and one for low water, if this

F

mode be more convenient). In this tube must be a float (a hollow box or ball, for example), which must carry an upright rod, or else must have attached to it a string which passes upwards over a pulley and is stretched by a weight; and the part of the rod or of the string which is outside the tube must carry an index, which shall mark on a vertical fixed scale the rise and fall of the float.

By making the tube close below, except one or more small openings, the motion of the waves will very little affect the float, and the true rise and fall of the surface may be observed with much accuracy.

4. It may happen that the moment of the highest or lowest water is difficult to determine, either with or without the tube, on account of the water, while near the highest or lowest, stopping or hanging still, without either rising or falling, or else rising and falling irregularly.

If there is a considerable time during which the water neither rises nor falls decidedly, note the moment when it ceases to rise, and the moment when it begins to fall, and take the time half way between these for the time of high

water.

5. Another method is the following:-At certain intervals of time near the time of high water, for example, every ten minutes, or every five minutes, let the height of high water be observed, say for half an hour or an hour, and from the height so observed pick out the highest for the high water, and note the height and the time; and in like manner for low water.

6. But the following is a better mode of dealing with observations thus made every five or ten minutes. Let a number of parallel lines (ordinates) be drawn at intervals corresponding to the intervals of observations, and bounded by a line perpendicular to them on one side (the abscissa), and on these lines (the ordinates) let the observed heights of the surface be set off (from the abscissa), and let a line be drawn through the extremities (of the ordinates). This line, if it be tolerably regular, will give the time of high water; and if it be somewhat irregular, it can be smoothed

into a curve, and then the time and height of high water read off. And in like manner for low water.

Suppose, for example, that we have the following observations of the height of the water made every five minutes for an hour:

m. m. m. m. m. m. m. m. m. m. m. m. m.

5 10 15 20 25 30 35 40 45 50 55 60

The selection of the greatest height (as in 5) would give high water at Oh. 30m.; but the general run of the height (6) would give the high water two or three minutes later, as appears by drawing the dotted curve in fig. 1.

This way of finding the exact time of high water (or low water) from observations made every five or every ten minutes, between some of which the highest water happens, is called "interpolating." See the article on Hydrography, Appendix No. 6, where we have the following observations, made at intervals of 10 minutes:

By interpolation from these observations, we find, for the exact moment of high water

7. It is easy to draw such curves, if we have, ready prepared, paper ruled into small squares, the divisions in the horizontal line representing hours and minutes, and

the divisions in the vertical line representing feet and inches.

8. It is well to begin a series of tide observations at any place by observing the height of the water, during the whole of the day and night, every half-hour or every quarter of an hour. For if the rise and fall be very irregular, or have any features which make it differ much from the common rule, it will, by this means, be seen that the case is a peculiar one, and that peculiar methods must be used: but if there is nothing peculiar in the case, the common methods may be used.

For instance, if, instead of there being two tides in every (lunar) day, there be one only, or four (both which cases occur at several places), these peculiarities will be discovered by observations continued during the day and night, in the way just recommended. If there be a periodical rise and fall of the sea's surface not depending in any obvious way upon the moon, the periods of maximum and minimum should be carefully and exactly observed, in order to determine upon what the rise and

Height of Water.