Z41 



THURSDAY. 



TIDAL HARBOURS. 



242 



distance than six leagues ; and if the clouds had been at, or a little 

 within, that distance, they must have been risible, unless it be sup- 

 posed that they were not more than a few yards above the ground ; 

 but the hail which followed the thunder must have proceeded from 

 clouds having great .elevation, though at the time the claps were heard 

 they were too remote to allow any sound from them to reach the ear ; 

 and therefore he concludes that the sounds must have been produced 

 in the air itself. 



From the meteorological observations made by Dr. Scoresby, and 

 Captains Phipps, Parry, and Ross, it appears that neither thunder nor 

 lightning is known to take place beyond the 75th degree of north lati- 

 tude ; even so low as the 70th degree those phenomena are very rare : 

 and in the tables of Captain Parry the occurrence of thunder and 

 lightning is mentioned but once between June, 1821, and September, 

 1823. Captain Franklin also, in 674 N- lat., heard thunder on one 

 day only between September, 1825, and August, 1826. 



THURSDAY. [WEEK.] 



THYMENE (C^H,,,). A hydrocarbon isomeric with oil of turpen- 

 tine, forming one of the constituents of the essential oil of thyme. 



THYMINE. An organic base found in the tht/mua gland of the calf 

 (sweetbread). It crystallises in needles, and also forms crystalline 

 salts. Its composition has not been determined. 



THYMOIL. [THYMOLE.] 



THYMOLE (C^H.jO, HO). Hydrate of Oiymyl. The oxidised por- 

 tion of the essential oil of thyme, forming about one-half of that essence. 

 It crystallises in oblique rhomboidal prisms, which are nearly insoluble 

 in water, but very soluble in alcohol and in ether. Under the in- 

 fluence of oxidising agents it yields a crystalline substance termed 

 thymail (C^H^O,), which is homologous with quinone. 



THYMYL-SULPHURIC ACID (C K U^O, HO, S.O.). An unim- 

 portant acid, obtained by the action of concentrated sulphuric acid 

 upon thymol. 



THYMYL-SULPHUROUS ACID. Synonymous with Sulpkocymollc 

 acid [CVMOLK]. 



TIA'RA (ridpa or Ttdpas), a high kind of hat, which was in ancient 

 times worn by the inhabitants of Middle and Western Asia, especially 

 by the Assyrians, Persians, Parthians, Armenians, and Phrygians. 

 There were two kinds of tiaras : the upright tiara was only used by 

 kings, priests, and other persons of the highest rank, and the upper 

 part had frequently the shape of a crown ; the tiara worn by other 

 people was of a soft and flexible material, so that it hung down on one 

 side, as in the case of the so-called Phrygian bonnet. (Hcsychius and 

 Suidas, . v. ridpa.) The tiaras of persons of high rank were of the 

 most costly colours, such as purple, and adorned with gold and precious 

 stones. 



In modern times the term tiara is applied to the head-dress of the 

 popes, which is worn on solemn occasions, and consists of a triple 

 crown. The peculiar form of the papal tiara has led some archteologists 

 to trace its origin back to pagan antiquity. Triple crowns bearing a 

 marked resemblance to the papal tiara are worn by the Assyrian kings 

 as represented on the slabs found at Nineveh by Botta and Layard. 



TIDAL HARBOURS. The enclosures for the protection of vessels 

 which are situated upon the sea shore, or the portions of rivers affected 

 by the rise of the tides, are usually known by the distinguishing name 

 of tidal harbours, on account of the peculiar arrangements adopted in 

 them in consequence of the variations of level in the waters, and of 

 the currents which may prevail in the offing. Tidal harbours may be 

 Jiuiiniy or dry, according to the rise of the tide, or the depth of water, 

 when no artificial basins are formed ; or they may be classified as 

 natural or ar/ijirial harbours, according to the configuration of the 

 coast, and the mode of construction adopted in them. 



A natural jl'iating harbour is one wherein there is at all times of the 

 tide a sufficient depth of water to maintain a vessel afloat ; a natural 

 dry harbour is one which ia left without water at low tides, and in 

 which the vessels are obliged " to beach," or " to take the ground," 

 that is to say, are left temporarily high and dry. Artificial harbours 

 present occasionally one, or both, of these conditions ; but as they are 

 usually formed in positions where the navigation is sufficiently active 

 to require great facilities for the commerce carried on in them, it is 

 rare that artificial tidal harbours are constructed without the forma- 

 tion of floating basins in which large vessels may be kept constantly 

 afloat. Dry tidal harbours are only of use for coasting traffic, or for 

 vessels of small burden. The outer harbours of ports of greater 

 importance are often in fact nothing but tidal harbours, dry at low 

 tides ; but in such cases they are accompanied by half-tide basins, and 

 inner harbours or docks, in which the level of the water is constantly 

 maintained at that of the high tide by means of lock gates, or pontoons. 

 Craving docks, gridirons, scouring sluices, warehouses, and other 

 appliances are added in such cases according to the local physical 

 conditions of the situation, to the nature of the trade, or to the fiscal 

 organisation of the country in which the harbours may be situated. 

 These details have already been discussed under DOCKS ; and they 

 evidently must be the same in all import harbours, whether tidal, or 

 otherwise; or whether the outer harbour be wet or dry. 



Upon an open coast, like that of the southern counties of England, 

 excepting between the Race of Portland and Selsea Point, the harbours 

 must all be tidal, and dry at low water, unless they should be placed 

 at the mouths of rivers, or at the head of deep inlets from the sea. 



ARTS ABD SCI. DIV. VOL. VIII. 



Milford Haven and Plymouth are instances of this favourable natural 

 configuration, and there is little necessity for the formation of closed 

 docks in either of them. Southampton and Portsmouth present very 

 great advantages also, and they are almost entirely devoid of the 

 inconveniences usually attached to tidal harbours, for vessels can lie in 

 the inclosed entrances to the floating basins of either of those ports 

 without " touching," unless when their burden is very great indeed. 

 In the case of ordinary tidal harbours, it therefore follows that, unless 

 there should exist near them a good sheltered roadstead, vessels cannot 

 safely " make " them, unless at high water ; nor can they ever become 

 really " harbours of refuge," which should be accessible at all times of 

 the tide. Havre and Liverpool are instances of tidal harbours pre- 

 senting tolerably favourable conditions of access in consequence of the 

 existence of roadsteads. Cherbourg is an instance of a tidal harbour 

 rendered safe by the creation of an artificial roadstead ; for though the 

 military port of Cherbourg has a sufficient depth of water to allow the 

 largest vessels to enter at any time of the tide, yet the commercial 

 port is left high and dry twice a day. Ramsgate, Dieppe, and the little 

 harbour of refuge (so called) at Port en Bessin are instances of the true 

 tidal harbours, with all their characteristics and all their inconveniences. 

 They can only be entered at high tide ; their jetties are so much 

 exposed, that if a vessel should happen to miss the entrance she 

 would almost infallibly be wrecked ; and they are all liable to be silted 

 up by the alluvial matters carried forward by the flood-tides on their 

 shores. [TIDAL WATERS AND CURRENTS.] 



Under PIERS, the modes of constructing and the principles of 

 designing the walls which inclose the spaces intended to form harbours 

 have been already discussed ; and it may, under these circumstances, 

 suffice to say that the area to be inclosed in a tidal harbour designed 

 to receive ordinary coasting vessels should not be less than six acres of 

 water surface ; and that the area should increase from this minimum 

 dimension in proportion to the depth of water and to the activity of 

 the commercial relations of the locality. In the case of tidal harbours 

 of refuge, the area must be proportioned to the number of vessels 

 likely to resort to them. Thus the tidal harbour of Southampton 

 Docks, in which there is 18 feet of water at low tides, has an area of 

 16 acres ; the area of the tidal basin of the new port of St. Nazaire, at 

 the mouth of the Loire, is about 22 J acres ; that of the tidal harbour 

 of Port en Bessin, is about 27 acres ; that of the outer harbour of 

 Ramsgate is about 304 acres. Even in dry tidal harbours it is 

 desirable that the extremities of the jetties should be carried out into 

 6 feet water at low neap tides ; for the minimum depth of water at the 

 entrance should never be less than 18 or 20 feet. 



It may, perhaps, be advisable to state that tidal harbours whose 

 entrances are exposed to be swept by a strong flood current should, 

 generally speaking, have the jetty against which the flood strikes 

 carried beyond the jetty on the opposite' side ; whereas in deep-water 

 harbours the relative lengths of the jetties are regulated by their 

 positions with respect to the direction of the prevailing wind. In fact 

 the extension of the jetty on the outside of the flood may give rise to 

 a local counter-current which would facilitate the entry into the har- 

 bour ; and in some respects it may even cause the tide to " stale," as 

 seamen say, or to remain for a short time at a constant height in the 

 inclosed space, in consequence of the resistance offered by the great 

 ebb-tide in the offing to the efflux of the small quantity of water in 

 the port. The various currents which prevail in the British Channel, 

 at the mouth of the Seine, and of the Southampton Water, may be 

 referred to as illustrations of these peculiar conditions ; and at Havre 

 they occur in such a manner as to render the tidal outer harbour of 

 nearly as great value as an ordinary floating harbour during two hours 

 of each high tide, whilst at the same time they create a strong current 

 setting into the tidal harbour during the flood, or precisely at the 

 period when vessels are entering. Sometimes, however, these local 

 currents give rise to bars or sand-banks, either across the mouth of the 

 harbours or on the down side (to the flood) of the main channel, in 

 consequence of the interferences they produce with the advance of 

 the alluvions carried forward by the flood. Thus, at Dieppe, Boulogne, 



w E 



Newhaven, Harwich, Harlingen, &c., bars exist at the mouths of the 

 tidal harbours, and they are usually of a very dangerous character, 

 although occasionally, as at the last-named harbour in the Zuyder Zee, 

 the bars may form natural breakwaters, inclosing shallow and imperfect 



