Connected with these peculiarities, there is also a singular effect produced on the di rection of the current in a tidal river. In the open ocean, the water merely rises and falls, there being no perceptible tidal current. Sailors are in the habit of associating the ces sation of currents, or " slack " water, with the occurrence of high and low water. This is the case in bays, but not in rivers, and it gives rise to some curious errors regarding the time of high-water in rivers. Thus it is sometimes said that it is high-water in the center of the Thames' channel long after it is high-water at the shore—an obvious absurdity. The truth is, the current does not cease simultaneously at the shore and in the mid-channel. At the mouth of a tidal river, the water runs upward for hours after high-water, and downward after low; and the same is true, in a less degree, at places higher up the stream.
When considerable alterations of breadth. or depth occur in the channel of a river, we find corresponding alterations in the amount of rise of the tide. Thus, according to Airy, at the entrance of the Bristol channel, the whole rise at spring-tides is about 18 ft.; at Swansea, 30 ft. ; and at Chepstow, 50 feet. At Annapolis, in the bay of Fundy, the tide is said to rise 120 feet. Again, the same port may be reached by tide-waves com ing from the ocean by different channels; and here we have to compound the two dis turbances just as we did with the separate lunar and solar tides. In the German ocean, we have a very good example; but the most remarkable is the tide at Batsham, in Tonquin. At this port, two tide-waves meet, coming respectively from the Indian and China seas; these bring, simultaneously, opposite but nearly equal changes in the water level, and the effect is, that there is almost np perceptible tide.
Whewell, Lubbock, and others have lately added much to our knowledge of the facts of the tides: and have constructed what are believed to be tolerably accurate charts of coticlal lines—that is, lines representing the positions of the crest of the tide-wave at hourly intervals as it sweeps round the earth. A great deal, however, remains to be
dcne in this direction, before we can hope to elicit from observation such hints as may enable us to improve the mathematical theory of the subject.
The frictional resistance to the motion of the tide-wave of course produces heat.• This heat is a transformation of part of the earth's energy (see FoncE) of rotation; and thus it appears that the tides are gradually lengthening the day. We may see easily that this would go on, were the moon the only tide-producing body, so long as the earth rotates about her axes in less time than a lunar month. For if the length of the (sidereal) day were that of a lunar month, the earth would always turn the same face to the moon; and the tide-spheroid would have a Axed position on the earth, and there would be no loss of energy by friction. Simple as this deduction is, though it seems to be roughly guessed at by Kant, it was not formally enunciated till about thirty five years ago. Mayer was the first to publish anything on the subject, but it seems to have been previously noticed by others. One of the most curious deductions from it is the recent speculation which assigns, as the cause of the moon's turning always the same face to the earth, the friction of the enormous tides which must have been produced by the earth in her mass when it was in a molten state, on the surface at least, if not throughout.
The only work with which we are acquainted from which complete information as to our present knowledge of the subject of tides may be obtained is that of Airy, in the Encyclopaedia .2ifetropolitana, already referred to.
TIE, in music, an arch drawn over two notes on the same degree, uniting them so that they are played or sung as one single note of the same value. Thus, for the two C's written in the example , one is played of the value of a minim and el quaver combined. The tie is often used in syncopated passages to connect the last note of one measure with the first of the succeeding one, when the former note, which would otherwise be unaccented, acquires the emphasis of the latter: • - =a= - 0 0