T I D 



[436] 



TIL 



TIDE, (eble and fluth, Germ, flusso 

 del mare, It.) The flow of the 

 water in the ocean ; the alternate 

 rise and fall of the sea twice in the 

 course of a lunar day, or in 24h. 

 50m. 28s. of solar time. The 

 tides are a subject on which many 

 persons find a great difficulty of 

 conception. As the tides depend 

 upon the action of the sun and 

 moon, their rise and fall are classed 

 among astronomical problems, of 

 which they are the most difficult, 

 and their explanation the least 

 satisfactory. That the moon, by 

 her attraction, should heap up the 

 waters of the ocean under her, 

 seems to most persons very natural, 

 that the same cause should, at 

 the same time, heap them up at 

 the opposite, appears to many a 

 palpable absurdity. Yet nothing 

 is more true, nor indeed more evi- 

 dent, when we consider that it is 

 not by her whole attraction, but by 

 the differences of her attractions, at 

 the two surfaces and at the centre, 

 that the waters are raised. In the 

 semi-diurnal tides there are two 

 phenomena particularly to be dis- 

 tinguished, one occurring twice in 

 a month, and the other twice in a 

 year. The first phenomenon is 

 that the tides are much increased 

 in the syzigies, or at the time of 

 new and full moon. In both cases 

 the sun and moon are in the same 

 meridian: for when the moon is 

 new they are in conjunction ; when 

 she is full they are in opposition. 

 In each of these positions, their 

 action is combined to produce the 

 highest or spring-tides under that 

 meridian. The neap-tides take 

 place when the moon is in quadra- 

 ture. The higher the sea rises in 

 full tide, the lower it is in the ebb. 

 The second phenomenon is the aug- 

 mentation in the tides occurring at 

 the time of the equinoxes, when 

 the sun's decimation is zero, which 

 happens twice every year. The 



greatest tides take place when a 

 new or full moon happens near the 

 equinoxes, while the moon is in 

 perigee. The height to which the 

 tides rise is much greater in narrow 

 channels than in the open sea, on 

 account of the obstructions they 

 meet with. The tides in the Brit- 

 ish channel sometimes, in some 

 parts, rise as high as fifty feet; 

 whereas on the shores of some of 

 the islands near the centre of the 

 Pacific ocean, they do not exceed 

 one or two feet. Theoretically, 

 all bodies of water, even large 

 fresh water lakes, have tides ; but 

 they are so insignificant that in- 

 land seas, such as the Mediter- 

 ranean and Black Seas, are gene- 

 rally termed tideless. One of the 

 most remarkable circumstances in 

 the theory of the tides is the as- 

 surance, that in consequence of the 

 density of the sea being only one- 

 fifth of the mean density of the 

 earth, and the earth itself increasing 

 in density towards its centre, the 

 stability of the equilibrium of the 

 ocean can never be subverted by 

 any physical cause. A general 

 inundation arising from the mere 

 instability of the ocean is therefore 

 impossible. HerscMl. 

 TIL'ESTONE. The name given to that 

 formation which constitutes the 

 third or lowest division of Sir 

 R. Murchison's Old Red system. 

 The tilestone, though of much 

 smaller dimensions than the over- 

 lying formations, has very marked 

 characters both in structure and 

 fossil contents, and is very clearly 

 defined by occupying a position in 

 which it "passes upwards into the 

 cornstone and marls, and down- 

 wards into the Silurian rocks. 

 The beds are finely laminated, 

 hard, reddish or green, micaceous, 

 quartzose sandstones, which split 

 into tiles. Although the greenish 

 colours prevail, these beds are 

 usually associated with reddish 



