Page:The American Cyclopædia (1879) Volume XV.djvu/773

 TIDES 743 singular way. In each of these basins the juatorial tide has to take a fresh start from the eastern side with every fresh upper and ' )wer transit of the moon and sun, and is de- royed or confused by reflection on the west- coast before the creation of a new wave ; rhile in the open part of the southern ocean tide wave circulates unimpeded, and spreads ito the three oceans up which it runs as a ree wave, from S. E. to N. W., overtaking in progress and compounding with the partial matorial tides or forced waves proper to ither ocean. On approaching the shore, the raves are elevated and retarded by the slope )f the bottom, and deflected or crowded to- other according to the varied configurations the coasts. It is owing to these complica- ions, together with our ignorance of the laws )f friction among the particles of water, and atween the water and the bottom, that our leories fail to inform us of the magnitude id time of the tides at any given place. But ley determine the periodicity of their phases, id the relative part which each disturbing >rce bears to the whole, by which we are ")led, by the analysis of a sufficient series of observations at any place, to predict the of the tides at the same place for any iture time, the knowledge of which is of im- lense importance to navigation. It is only ice the beginning of the present century that le science of the tides has made any consid- erable progress in this direction. The theo- retical investigations of Laplace, in the Meca- lique celeste, and his discussions of the tidal )bservations at Brest, opened the way. Lub- 3k and Prof. Whewell contributed large- by the elaborate discussions of large collec- ions of tidal observations, published in the " Philosophical Transactions " of the royal society ; and Prof. Airy, in his essay on " Tides and Waves " in the " Encyclopaedia Metropol- itana," has greatly extended our theoretical conceptions of the subject. More recently still important investigations have been published by Prof. W. Thomson and Mr. W. Ferrel. The tides on the coasts of the United States have been specially investigated by the late Prof, jhe as superintendent of the American coast rvey. In connection with that work he organ- 3d an extensive system of exact observations, for the purpose of ascertaining the complicated laws which govern the tides. It will be readily understood that in order to separate the effects of the different causes which modify the phe- nomena, it is not sufficient to observe merely the heights and times of high and low water, but "mt a continuous record of the tides is neces- ry, as the inequalities are constantly shifting their place and magnitude. For this purpose self-registering tide gauge is used, by which continuous curve representing the successive changes in the height of water is traced on paper moved by clockwork, by a pencil acted on by the rising and falling of a float in a ver- tical box, to which the tide has free access. The time scale is such that every hour is rep- resented by one inch, and is pricked into the paper by points on the cylinder which moves the paper forward. A continuous sheet, suffi- cient for the record of a whole month, is put on 'the tide gauge at one time. A complete description of this instrument will be found in the coast survey report for 1853. Prof. Bache gave in his annual reports on the progress of the coast survey, from 1851 forward, a series of papers on the tides, detailing the processes of discussion, and giving the results as they were from time to time developed. In these are con- sidered the apparent anomalies in the tides in the gulf of Mexico, exhibiting at some places only one tide in 24 hours ; the large inequali- ties in the tides on the Pacific coast ; the gen- eral progress of the tide wave along our coasts and in the bays and rivers ; 'the influence of the winds in particular localities; and the action of tidal currents on the bars and chan- nels of our harbors. These labors, which are still in progress, have resulted already in the annual publication of " Tide Tables," giving in advance the times and heights of high and low water at all the principal ports of the United States, for every day in the year. An elabo- rate discussion of the tides observed at Boston and New York during 19 years, a full lunar cycle, has been made by Mr. "William Ferrel of the coast survey, and has resulted in repre- senting the actual tides with unlooked-for pre- cision, yielding moreover a value for the mass of the moon closely approaching that obtained by astronomical methods. The tides on the coast of the United States, on the Atlantic, gulf of Mexico, and Pacific, are of three differ- ent classes. Those of the Atlantic are of the most ordinary type, ebbing and flowing twice in 24 hours, and having but small differences in height between the two successive high or low waters, one occurring before noon, the other after noon. Those of the Pacific coast also ebb and flow twice during 24 hours, but the morn- ing and afternoon tides differ very considerably in height, so much so that at certain periods a rock which has 3 ft. of water upon it at low tide may be awash (nearly bare) on the next suc- ceeding low water. The intervals, too, between successive high and successive low waters may be very unequal. At San Francisco, for exam- ple, at a time when the moon has a large south- ern declination, the high water occurring about 12 hours after the moon's transit may mark 5 ft. on a tide staff; five hours afterward low water will mark 3 ft., six hours after which the second high water will reach 7 ft., and seven hours later the second low water will fall to zero. These inequalities depend upon the moon's declination, in the manner which we have explained ; they disappear at the time of the moon's declination being nothing, and are greatest about the time of its being great- est. These tides exhibit the normal type, while those at New York and adjacent parts of the Atlantic coast do not exhibit the diurnal in-