BOBK 



nORINO INSTRUMI-ATS 



feet, and form* three or even (bur swells, which follow in rapid succes- 

 iuu. fRMAXOXAS, in QEOO. Div.] It U also said that some puti of 

 these rivers being obstructed by shoals, the pororoca U only observable 

 on these shoal*, and that it disappears in deep water, for which reason 

 the barge* are moored in theae parts, where they are only exposed to 

 a strong agitation of the water*. (RenneU's 'Hindoostan; Ayre's 

 Corografia Bnudlka ; ' and Eschwege a ' Bnailien.') 



The preceding statements comprise the principal fact* embraced in 

 the local history of this phenomenon, aa described by various obaerven, 

 and at various period* ; but recent investigations, by which the theory 

 of the tidea and of wares haa been at once corrected and worked out 

 into detail, enable ua to add a more explicit view of the nature of Uie 

 bore, which baa become a subject of great theoretical interest 



The Her. Dr. Whewell haa ahown that we may consider the banks 

 uf a river as a portion of the aea-ahore in which there U a point ol 

 convergence of tides ; and the magnitude of the tides will be increased 

 by this convergence. If we consider the production of theae very high 

 tidea of convergence aa a mechanical question, they may be accounted 

 tor by what is called " the principle of the conservation of force." 

 When any quantity of matter is in motion, its motion is capable of 

 carrying every particle of the mass to the height from which it must 

 hare fallen to acquire its velocity ; but if the motion be employed in 

 raking a nmlUr quantity of matter, it is capable of raising it to a height 

 proportionally greater. In channels which narrow considerably, the 

 quantity of water raised in the narrow part is less than in the wider, 

 and thus the rise in such cases is greater. The tide being thus made 

 to rise greatly by the contraction of its channel, the part of the water 

 so aflected may be abruptly terminated on the inland aide, the depth 

 and quantity of water on that side not allowing the surface there to be 

 immediately raised by means of transmitted pressure. A tide-wave 

 thus rendered abrupt, is called a bore, and has a close analogy with the 

 waves which curl over and break on a shelving shore. It would appear, 

 indeed, from the researches of Scott Russell, that such waves present 

 us with the limit of the phenomenon. The bore itself, as we have seen, 

 occurs in many places in considerable magnitude, appearing to travel 

 with great rapidity, though moving slower, in fact, than the tide-wave 

 under any other circumstances ; and producing great noise by the 

 huge amount of intestine motion of its particles. 



One of the results of Mr. John Scott Russell's experimental deter- 

 mination of the nature of waves, just cited, is the identification ( whi.-h 

 had been anticipated by Dr. Whewi-11) of the tide- wave with that which 

 he has denominated the great primary wave of translation [ W.vvr.]. it 

 being the only wave of the ocean belonging to that order. The 

 tide appears to be a compound wave, one elementary wave bringing 

 the first part of flood-tide, another the high water, and so on ; these 

 move with different velocities according to the depth. On approach- 

 ing shallow shores, the anterior tide-waves move more slowly in 

 the shallow water, while the posterior waves moving more rapidly, 

 diminish the distance between successive waves. The tide-wave 

 thus becomes dislocated, its anterior surface rising more rapidly, 

 and its posterior surface descending more slowly than ifl deep water. 

 According to the principles thus established by Mr. Scott Russell, " a 

 tidal bore is formed when the water is so shallow that the first waves 

 of flood tide move with a velocity so much less than that due to the 

 ninooofling part of the tidal-wave, as to be overtaken by the subsequent 

 waves, or wherever the tide rises so rapidly, and the water on the 

 shore or in the river is so shallow, that the height of the first wave of 

 the tide is greater than the depth of the fluid at that place. Hence in 

 deep water vessels are safe from the waves of rivers which injure 

 those on the shore." This last consequence is indicated in the accounts 

 of the local phenomena. (Win-well, ' Essay towards a First Approxi- 

 mation to a Hap of Co-tidal Lines.' Phil. Trans., 1833, pp. 203205 ; 

 Scott Russell, ' Report on Waves,' Rep. of Seventh Meeting of British 

 Association, 1837, pp. 426, 427.) In some observations made in 

 the Section of Mechanical Science of the same meeting of the Asso- 

 ciation, Mr. Russell applied his theory of the tide-wave to the improve- 

 ment of riven by the removal of the bore, which he stated, "is 

 principally found in rivers and places of rivers where the level of the 

 bed is situated near the level of the mid-tide. This wave changes its 

 form as it approaches a shallow coast, the top being then accelerated, 

 and breaks in a surge when it gets to water of less depth than itself. 

 Supposing, then, that the tide has a bore three feet high, the water 

 being three feet deep, by ili>e|isiiliiii the channel you remove the bore, 

 and this is the only effectual remedy." (' Athenaum,' 1837, p. 704.) 



On reviewing the present state of knowledge and theory on the 

 subject of the bore, as a topical condition of the tidal-wave, it is 

 manifset that science still requires an exact investigation of particular 

 cases, as occurring in localities differently circumstanced, giving 

 precise measures, both of the heights and velocities of the wave, and 

 an application of the theory to explain the varying particulars of the 

 phenomenon ; together with an explicit verification of such portions of 

 it as, though in the highest degree probable, must still be regarded a* 

 l.i !.. . 



This is confirmed by the following comprehensive description of the 

 bore by the Astronomer-r yal, Mr. Airy, which may be regarded as a 

 commentary both on the local accounts and the theoretical explanations 

 given above. Connected with the rapid rise when the tide is very large 

 in proportion to the depth of the water, " is the bore ; a phenomenon 



on which, we believe," Mr. Airy remarks, " some misconception pre- 

 vails. We believe that the following drwriptioii of its cause and 

 appearance will be found ...M,V, It is necessary for its formation 

 that there be a very large tide rising with great rapidity (thus at 

 Xewnham [on the Severn], where the water rises 18 feet in an hour 

 and a half, the bore is considerable). It is necessary, also, that tin- 

 channel of the river be bordered with a great extant of flat sands near 

 to the level of low water. These circumstances hold in the S 

 the Seine, the Amaconas, the bays at the head of the Bay of Fun.ly 

 (Chignecto Bay, and the Bay of Mines), and other places when- the 

 bore is remarkable : the second does not In .1.1 in the Thames; ami. in 

 consequence, that river has no bore. When the rise of the tide 

 the surface of the water is disturbed in mid-channel, so as to 

 reflection; but the water is not broken, it is merely like a common 

 wave. This point (which we state from our own observation) is fully 

 confirmed by De la Condamiue's account of the bore in the Amaconas, 

 and by the practice in the Hoogly river (near Calcutta), and other 

 places subject to a bore, of rowing boats, &c., into tin- mi.Mle of tin- 

 channel on the approach of the bore, in order to place them out ..f 

 danger. But as this rapid rise elevates the surface suddenly above the 

 level of the flat sands, the water immediately rushes over them witli 

 (jreat velocity, and with a broken front, making a great noise. Ami 

 this is the whole of the bore. It is, however, a majeMi.- phenomenon, 

 especially when witnessed from a station which commands the river for 

 several miles above and below (we may particularly mention Newnham 

 churchyard, on the Severn), and at on hour when other sounds are 

 stilled. The rise of the water continues, after the bore has passed, 

 with unabated rapidity as far as can be seen by general observation, 

 and the tidal current flows rapidly up the river, which U now quite 

 full. At last its rapidity of rise diminishes, and at 90 minut. 

 the bore, at the place which we have mentioned, it begins to drop, tin- 

 current still flowing up. About 15 minutes after the beginning of 

 the drop, a singular line of ripple (of which we can give no further 

 explanation) is seen, stretching across the whole river, and moving 

 very slowly downwards. This appears to be the place at which the 

 ascending current and the descending current meet ; for, as soon as it 

 has passed, the water is seen to be running slowly downwards, ami in 

 a very short time it is running with a speed which is scarcely to In- 

 seen under any other circumstances of any river, except perhaps some 

 of the largest rapids. The remarkable point attending this Lastphe- 

 nomenon is, that the phase of change of current moves down the river. 

 In every other instance, the phases, whether of high water, of Ion- 

 water, or of change of current, move upwards ; although that of low 

 water moves upwards much more slowly than that of high water. It 

 would seem here that one of the velocities has actually changed its 

 sign." (' Tides and W;n Met.,' 514.) 



In a paper on the mathematical theory of sound, by the Rev. S. 

 Earnshaw, read before the Royal Society on the 6th of January list. 

 (1859), the phenomenon of the bore is investigated in its most simple 

 form, but in harmony with the more complicated cases of the waves of 

 water occurring in nature, and noticed in this article : " It is shown 

 that all the parts of a wave [in this cose in a uniform elastic medium ] 

 do not travel at the same rate ; a circumstance which leads to the 

 formation of a bore in the front of the wave." The principle is then 

 applied to account for several acoustic phenomena, previously un- 

 explained, such as double reports of fire-arms heard at a great di 

 and the outrunning of one sound by another, observed by Copt. Parry. 

 ('Proceedings of the Royal Society,' vol. ix. p. 5!H.) It does not 

 appear that the analogous condition of the undulations of tli.-.tli.r. 

 giving rise to optical phenomena, has been investigated, or even recog- 

 nised. As the production of the bore results from circxim < 

 foreign and external to the intrinsic nature of waves, it would seem to 

 require for its discovery an artificially constructed system of crj'Rt 

 bodies, having different refractive and polarising powers for light ; 

 though it is not improKible that systems of such a kind as would pro- 

 duce the phenomena may exist in nature. 



On the subject of the origin and etymology of bore we have no 

 express information. It has been conjectured to be an Indian won I, 

 but for this there is no foundation ; an.l.ii.i i. j.i.J -,M,- d.-riv..: 

 from the Anglo-Saxon, in the older sense of the English word In bore, 

 signifying to penetrate by a powerful forward action. When called 

 the Boar, and the Jtonr't heati, as is also the case, a comparison with 

 the impetuous rushing career of that animal is suggested, in which the 

 word may have originated. It may be doubted, however. \\ I 

 most, if not all, the terms designating this phjnomsBCfi, are not imitative 

 of the sounds which it produces, and have been involuntarily suggested 

 by them, as occurring in varying intensity and order. The prolonged 

 sound of bore, and its Kn-m-li form larrt, and the Brazilian jmrortca, 

 are strikingly of this character, which may possibly be attributable to 

 Hyijre also, given without etymon in our archaic dictionaries ; but 

 this is evidently the same word with the DuU'li cry /.;, denoting thu 

 kindred phenomenon of double high-water. Some of these terms may 

 subsequently have been employed in conformity with a suppcm. 

 mology, and if such has been the case, as in other cases of the confluence 

 .f meaning in philology, further ii|uiry would be hopeless. 



BORING INSTRUMENTS. Whether the vertical cavity for an 

 artesian well is made, or the cavity of a cannon formed, or the cavity 



