TIDES equality. The explanation of this feature is probably to be found in the supposition that the tide wave which advances up into the At- lantic ocean from the continuous tide in the Southern ocean, arrives on our shores 24 hours later than the direct tide wave which crosses the Atlantic from E. to W. In this way the diurnal inequality will be eliminated by the superposition of the two tides, the greater high water of the former coinciding with the lesser of the latter, and vice versa, leaving the semi- diurnal tides of equal height. The tide at Galveston, in the gulf of Mexico, furnishes a case of the elimination of the semi-diurnal tide, leaving only the diurnal inequality. It is to be presumed in this instance that the tides reaching Galveston through the straits of Flor- ida and through the passage between Cuba and Yucatan differ oy six hours in their periods, causing the low water of one to coincide with the high water of the other r thus sensibly de- stroying the semi-diurnal tides, except in so far as they are unequal. This leaves a small tide outstanding, having substantially the form of the diurnal inequality, and producing the ap- pearance of the " single day tide," or one high and one low water in every 24 hours. This residual fluctuation is well marked at times when the moon's declination is considerable on either side of the equator, but disappears almost entirely when the moon is near the equator, since at such times the diurnal in- equality disappears. Tides of this class have always a small range ; in the gulf of Mexico they rarely exceed 2| ft., and the average rise and fall is but H ft. The tide gauges being in continuous operation, all other fluctuations of the ocean level, besides that produced by the tides, are likewise registered. The tide curves of the western coast are frequently found in- dented by fluctuations arising from earthquakes. A remarkable instance of this kind was fur- nished by the earthquake that destroyed* the city of Shimoda, Japan, in December, 1854. The time required for the transmission of the sea waves from Shimoda to San Francisco was 12h. 36m. The distance being 4,500 m., the trans- mission of the wave was at an average rate of 860 m. an hour. The theory of wave motion teaches us that this velocity will be attained by a free-moving wave in a depth of 1,440 fath- oms, which may be taken as the average depth of the Pacific between Japan and California. The crests of the waves occurred at intervals of about 23 minutes, corresponding to a length from crest to crest of 150 m. The height when the waves arrived at San Francisco was about 18 in. from hollow to crest. The great earthquake in Peru in August, 1868, was likewise recorded on the tide gauges at San Diego, San Francisco, and Astoria. The fluctuation of the ocean in this instance was very sensible to casual obser- vation, and was noted in Australia, at the Sandwich islands, and at Kodiak, Alaska. The data obtained from these observations, com- bined with the result before mentioned, indi- cate that the average depth of the Pacific ocean is about 1,800 fathoms. Such waves, origina- ting with an impulse at one definite point, and propagated freely through the ocean in every direction with a velocity depending upon the square root of the depth of the sea, serve as good illustrations of the manner in which tides are propagated as free waves through sounds, bays, and rivers. The rate of motion for differ- ent depths is as follows : at 10 ft., 12'2 m. an hour ; 60 ft., 30 m. ; 100 ft., 38'7 m. ; 1,000 ft.. 122-3 m. ; 6,000 ft., 299'5 m. TIDE TABLE FOR THE UNITED STATES.* PORTS. Mean luni- tidal In- terval. Rise anJ fall, spring tides. Rise and fall, neap tides. Eastport, Me h. m. 11 8 11 15 11 25 11 23 11 22 10 57 11 18 11 12 11 27 11 19 11 5 11 22 11 58 12 24 12 22 12 16 11 43 8 4 7 59 834 745 7 81 7 36 7 40 748 7 59 7 57 745 7 82 7 86 8 20 7 29 8 13 9 19 9 57 10 8 11 2 12 84 13 50 1549 16 55 17 48 9 9 7 9 88 9 28 11 16 11 11 11 7 11 13 11 22 11 20 7 32 8 19 8 8 33 9*4 9 52 11 58 13 44 8 17 feet. 20-6 9-8 9-9 9-9 ' 9-1 10-2 10-6 10-9 11-3 11-4 18-2 10-8 5-3 8-6 8-9 2-5 1-8 2-8 4-7 2-0 8-9 4-8 8-8 4-2 5-0 5-3 4'6 4-6 3-7 8-5 2-4 56 5-4 4-4 4-0 8-8 8-2 8-9 4-6 4-4 8-0 2-5 8-1 8-2 2-9 3-1 6-2 8-0 9-2 8-9 8-6 9-2 5-4 6-0 4-5 6-2 7-0 6-9 6'9 6-8 8-0 feet. 15-4 7-0 7'6 7-2 6-6 7-1 7'6 8-1 8'5 9-0 9-2 7-7 2-6 2-6 1-8 1-i 1-8 1-8 8-1 1-2 1-8 2-9 2-8 2-9 8-7 8'5 2-8 8-1 2-6 2-0 1-8 4-0 8-4 2-7 2-7 2-5 2-0 2'4 8'2 8-0 2-8 1-9 2-4 2-2 2-3 2-1 5-2 4-7 5-4 6-4 6-6 6-1 8-6 4-3 8-0 8-9 5-1 5-0 6- 5-1 2-0 Hanniwell's Pt., Kennebec river. Me. Portland, Me Portsmouth, N. H Rockport, " Salem, " Boston light, " Boston, Plymouth, Wellfleet, Nantucket, Hyannis, Edgartown, Holmes's Hole, Tarpaulin Cove " . Wood's Hole, N. side, Mass. . . . Wood's Hole, 8. side, " Menemsha Bight, " . 'Quick's Hole, N. side, " .... Quick's Hole, S. side, " Cuttyhunk, " ... Kettle Cove, " .... Bird island light, " New Bedford entrance, " . Newport, JR. I Point Judith, E. I Block island " Montauk Point L. I. N Y Sandy Hook, " . New York " Dobbs Ferry, Hudson river, N. Y. . Tarrytown, " . . Verplanck's Point, " . . West Point, ' " . . Poughkeepsie, Tivoli, " .. Stuyvesant, " . . Castleton, " .. Greenbush " . . Watch Hill R I Stonington Conn Little Gull island, N. Y New London, Conn New Haven, " Oyster Bay L. I., N. Y Sand's Point, " " . . New Rochelle,N. Y Throg's Neck " Cold Spring inlet N J Cape May landing, " Delaware breakwater, Del Higbee's, Cape May, N. J Kgg island light " New Castle " Philadelphia, Pa Old Point Comfort, Va
- The mean interval in column 2 has been increased by 12h.
26m. (half a mean lunar clay) for some of the ports in Hudson river, Delaware river, and Chesapeake bay, so as to show the succession of times from the mouth.
