TIDES. 55 



the crest of one and the hollow of the other arrive simultaneously, 

 and neutralize each other. 



The tidal range at many points around our own coasts, as 

 well as in other parts of the world, is much affected in a similar 

 manner, although not to so great an extent. 



Tidal Establishment. On the days of new and full moon, the 

 moon passes the meridian at 12 o'clock (nearly), and the time of 

 high water at any port on such days is called the " establish- 

 ment " of that port. 



The approximate time of high water on any other day may 

 be found by adding the " establishment " to the time of the 

 moon's passage over the meridian on that day. 



In order to accurately determine the establishment of any 

 port an important point, both in regard to sea- works and navi- 

 gation a series of careful tidal observations must be taken. 

 With the view of eliminating error, or equalizing irregularities, 

 these should be plotted in diagram form (see Figs. 10, 11). 

 Ready-made diagrams, obtained by means of self-recording tide- 

 gauges, are, however, much to be preferred. 



The time of high water on the days following new moon, full 

 moon, first quarter, and third quarter, may be pretty accurately 

 ascertained by adding the "establishment" to the time of the 

 moon's passage over the meridian; but from new moon to first 

 quarter, and from full moon to third quarter, high water occurs 

 somewhat earlier than would be indicated by the above process ; 

 and from first quarter to full moon, and from third quarter to 

 new moon, it occurs later. 



Owing to the earth not being truly spherical, to the irregular 

 distribution of land and water, and to the great variations in 

 the depth of the latter, the calculation of the time of high water 

 for any port is one of some difficulty, requiring the use of the 

 higher mathematics. Sir William Thomson (now Lord Kelvin) 

 has, however, invented two machines, one of which he terms a 

 tidal harmonic analyzer, and the other a tide-predictor, their 

 object being to use his own words "to substitute brass for 

 brain in the great mechanical labour of calculating the elementary 

 constituents of the whole tidal rise and fall." 



These machines perform their work in the most perfect 

 manner. All that is required is to furnish the operator with 

 a set of tidal observations, or the curves drawn by a self- 

 recording tide-gauge, for any particular port ; care being taken 



