740 TIDKS the Atlantic COast of the Tinted States at. | all direct ions t oward t he opening, w it bout re- H.iM.ras, Naiiluckel island, and (-ape moving much of the deposit, and gradually Sable making an allowance for the difference ' concentrates in definite narrow channels, v hich j,, | ,.;,! | Hue. If hv a line on the map w c con it scoops out, and the depth of which will de- ,,,,,.( these point- at which high water occurs pend in a freal decree on (In* proporl ion of simultaneoiislv , we mav regard that line as t he area of the basin to I he out let, or, in ot her represent ill." the crest f a tide wave advan em ; upon I he coast. U e shall I'M id high vv aler to occur later and later as we go up ml ' reached (luring the ehl> het eeii the I he ocean, vliich determines the d by following up the pro- locity and transporting power reach..! hv ill barred liarh bav.s and rivers; .re,, of the wav lilies ivpre.elltil the lop of the w a e for each si Such lines are called co tidal lines, and have changed during violent storms, when the di- been traced for the coasts of the Pniled Slates ivclion of the waves happens to be oblique to thai of the channel; or when the sea runs directly upon the channel, the depth of water m. M be considerably diminished for the time being, by the sand rolled up bv the waxes. Mill in all these eases mal depth is speedil; of tht ebb tide, which depends upon the un- changed factors of area and form of basin, height of tide, and character of the material forming the bar. To illustrate the important d currents, we w ill examine the m of New York hai bor. Con- I he progress of (he I ide w av e through Pong Island sound from the eastward to its meeting with that entering' New York bav at Sandv Hook, w e see that about YA hours bv Prof. Mache, for w hich w e the coast lines for the Mrili h isles, by Prof. hewell, w ill he found ill Keith John .|,,n' , " Phv sieal Atlas," as well as a chart of co tidal lines for Mic hole "lob,-; but (he latter must be looked upon as H rather adventurous generalization, in the absence of any positive know ledge of the tides in mid-ocean. The tides about the Mril ish isles present a verv inlere-.t in."; .ludv. The advancing high water p.r ics up the PirMr-h channel, oceiipv ing si hours from the Seilly i les to the mouth of the Thames, where it is m -t and recnforced by the high water P 1 hours older, which has travelled around the isles to the northward and down the North sea. Thi' re is a point in the latter, between Yarmouth and the Tc 111- ithin Mlock island with a co-tidal line of nine hours of (bo hitler title elevation of '.! ft., and at tin wave intersects that of three hours of the for jus) passed Sandy Hook with an elevation of mer, causing the interference of low water of Pill. Traversing the sound west ward with iil- the one with hi:'h water of t'ie other tide, in creasing hei-hts, it reaches Sand's Point three hours later with a height of 7'7 ft. The ob- served time of transmission from the Pace to Sand's Point is 'Jh. Im., and the time com- eonsequenee of which no change, takes place in the sea level, as has been ascertained bv actual observations over a shoal spot in that locality. A remarkable ease of the meeting of t u o tides, which will be more particularly no- puled from the depths according to the law developed by Airy is 'Mi. llm. ; a erv rood liced below, occurs near I'liroy/s Neck at the approximation when we consider the irrtru- V. ond of Long Island sound. The agency of larities in the configuration of the sound, which tidal currents in producing changes in the en | could not be taken into account. Advancing trances of havsand harbors is a subject of the still further, the height somewhat declines in lii'st importance to commerce and navigation, consequence of the changes of direction in the which has received full attention in the prose channel and its shallow ness. At Hell (iate eution of the American coast survev. As on this tide wave is met by that which had en- the average the same amount of water moves tere.lat San.lv Hook, ami advanced more slow- 'nward and outward with the llootl and ebb Iv owing to the narrow ne-s and intricacies of tides, we might readily suppose that ihe same- the channel, especially in the Kast river. These amount of material is transported either way, two tides which meet and overlap each other autl that no important change would take place at Hell (iate, differing in times and heights, in the contigurat ion of the bottom. Hut the cause contrasts of water elevations between operation of the llood stream is very dilVerent the sound and harbor which call into existence the violent currents that traverse the Kast river. The conditions of the tidal circulation through Hell (iate are such that if there were a par- it, the water would sometimes from that of the ebb stream. Vc have as a general feature an inti lent communicating w parativ dv narrow pass therefore, running with considerable velocity stand nearly ."> ft. hiHier, and at other times tlirough this channel, will as it enters the basin , r ft. lower on the one side than on the other. .-1 out and become slow, deposit in;-; the In the actual case of the superposition or eoin-
- .md and mud it is charged with, and making pounding of the two titles, the difference of
'xtensie llats or shoals opposite tlu> entrance, level existing at anv tinu 1 is tif course much The ebh stream runs sKnvly over the tlats from U'ss, but the dilYcrcnee of one foot is often
