WAVES. 



mined, bciiij^ frequently dclitieut one-ciglitfi or one-tenth 

 <KiIy of the whole. In other cafes, where a number of fmall 

 waves follow each other at intervals confiderably lefs than 

 the depth, he has endeavoured to calculate the retardation 

 which muft be occafioned by the imperfeft elafticity or com- 

 preffibility of the fluid ; but it feems probable that the 

 motion of fmall waves is ilill much flower than this calcula- 

 tion appears to indicate. Many of the phenomena of waves, 

 fays this ingenious author, may be very conveniently ex- 

 hiljited by means of a wide and (hallow velfel, with a bot- 

 tom of glafs, terminated by fides inclined to the horizon ; in 

 order to avoid the confufion which would arife from the 

 continual refleftions produced by perpendicular furfaces. 

 The waves may be excited by the vibrations of an elaftic rod 

 or wire, loaded with a weight, by means of which its mo- 

 tions may be made more or lefs rapid at pleafure ; and the 

 form and progrefs of the waves may be eafily obferved, by 

 placing a light under the veffel, fo that their fhadows may 

 fall on a white furface, extended in an inclined pofition. 

 In this manner, the manifeft inflection of the furface of the 

 water may be made perfeftly conipicuous. 



This motion of the fea-water depends greatly on the 

 winds, and on the fituation of mountains, in regard to the 

 fea ; for the winds are driven back from thefe with great 

 impetuofity ; and in fome places this occafions a great and 

 very irregular undulation, befide that which is produced by 

 the immediate aftion of the winds on the furface of the 

 water in ihcir own dirett courfe. 



Waves are to be confidered as of two kinds, and thefe 

 may be diftinguilhed from one another by the names of 

 natural and accidental waves. 



The natural waves are thofe which are regularly propor- 

 tioned in fize to the ilrength of the wind, whofe blowing 

 gives origin to them. The accidental waves are thofe occa- 

 fioned by the wind's readying upon itfelf by repercuflion 

 from hills and mountains, or high fhores, and by the wafh- 

 ing of the waves tlienif'lves, otherwife of the natural kind, 

 againft rocks and ftioals : all thefe caufes give the waves an 

 elevation, which they can never have in their natural fl:ate. 



It feems to be pretty well determined, from a variety of 

 experiments and obfervations, that the moll violent wind 

 never penetrates a great way into the water ; and that in 

 great ftorms the water of the fea is flightly agitated at the 

 depth of 20 feet below the ufual level, and probably not 

 moved at all at the depth of 30 feet, or 5 fathoms ; con- 

 fequently the natural difplacing of the waters by the wind 

 cannot be fuppofed to reach nearly fo low : and hence it 

 fiiould feem, that the greateft waves could not be fo very 

 high as they are reprefented by accurate and creditable navi- 

 gators. Mr. Boyle found, upon inquiry, that when the 

 wind was high, fo that the waves were manifeftly fix or feven 

 feet high above the furface of the water, no fign of agita- 

 tion was perceived at the depth of 15 fathoms ; but if the 

 blaft continued long, then the mud at the bottom was itirred, 

 and the water became thick and dark. Perfons who have 

 remained for fome time at a confiderable depth have been 

 furprifed to find a florm, when they have returned to the 

 upper parts of the water. At the depth of 12 or 15 feet, 

 the agitation of the water has been accordingly diminiflied ; 

 and at a confiderable diltance from the fiiore, and in deeper 

 water, the fea has been found proportionably calm and tran- 

 quil, it has alfo been found, that ni a part of the fea often 

 tempeftuous, the llorm did not reach with any efficacy four 

 fathoms beneath the furface of the water. Boyle's Works, 

 vol. iv. p. 354, 4to. Relations about the Bottom of the 

 Sea, § 3. 



Count Mariigli nicaiured carefully tiic elevations of the 

 waves near Provence, and found that, in a very violent tem- 

 ped, they arofe only to feven feet above the natural level of 

 the fea ; and this additional foot in height, above the refult 

 of Mr. Boyle's deduftions, he eafily refolved into the acci- 

 dental (hocks of the water againfl; the bottom, which wa?, 

 in the place he meafured them in, not fo deep as to be out 

 of the way of affeding the waves ; and he allows that the 

 addition of one-fixth of the height of a wave, from fuch a 

 difturbance from the bottom, is a very moderate alteration 

 from what would have been its height in a deep fea ; and 

 concludes, that Mr. Boyle's calculation holds perfeftly right 

 in deep feas, where the waves are purely natural, and have 

 no accidental caufes to render them larger than their juft 

 proportion. ' 



In deep water, under the high (hores of the fame part of 

 France, this author found the natural elevation of the waves 

 to be only five feet ; but he found alfo that their breaking 

 againft rocks, and other accidents to which they were liable 

 in this place, often raifed them to eight feet high. 



We are not to fuppofe, from this calculation, that no 

 wave of the fea can rife more than fix feet above its natural 

 level in open and deep water ; for waves immenfely higher 

 than thefe are formed in violent tempefts in the great feas. 

 Thefe, however, are not to be accounted waves in their 

 natural (tate, but they are fingle waves formed of many 

 others ; for in thefe wide plains of water, when one wave is 

 raifed by the wind, and would elevate itfelf up to the exaft 

 height of fix feet, and no more, the motion of the water is 

 fo great, and the fucceifion of waves fo quick, that, during 

 the time this is rifing, it receives into it feveral other waves, 

 each of which would have been at the fame height with itfelf: 

 thefe run into the firfi; wave one after the another, as it is 

 rifing ; and by this means its rife is continued much longer 

 than it naturally would have been, and it becomes accumu- 

 lated to an enormous fize. A number of thefe comphcated 

 waves arifing together, and being continued in a long fuc- 

 ceflion by the continuation of the ftorm, makes the waves fo 

 dangerous to ftiips, which the failors in their phrafe call 

 mountains high. Marfigli, Hift;. Phyf. de la Mer. 



When it blows fre(h, the waves not moving with fuffi- 

 cient rapidity, their tops, which are thinner and lighter, are 

 impelled forward, broken, and changed into a white foam, 

 particles of which, called the " fpray," are carried a great 

 way. Waves, with regard to their form, are circular or 

 ftraight, or otherwife bent, according as the original im- 

 preflion is made in a narrow fpace nearly circular, or in a 

 ftraight line, or in other configurations. In open feas the 

 waves generally are in the (hape of ftraight furrows, becaufe 

 the wind blows upon the water in a parallel manner, or at 

 leaft for a long apparent traft. The fame caufes wliich 

 raife water into waves muft evidently produce the like effeft 

 on other fluids, but in various degrees, as the fluid is more 

 or lefs heavy, as its particles adhere more or lefs forcibly to 

 each other, and probably likewile as there is a greater or 

 lefs degree of attraflion between the fluid and the other 

 body which gives it the impulfe. If it be attempted to 

 raife waves upon oil by the force of wind, it will be found 

 verv difficult to fucceed in a fimilar degree. This difficulty 

 is probably owing to the natural attradlion of the particles 

 of od ; and befides, there may be lefs attratfion between oil 

 and air than between the latter and water, for water always 

 contains a certain quantity of air ; and if it be deprived o£ 

 that air by boiling, or otherwife, a (hort expofure to the 

 atmofphere will enable the water to reimbibe it. It is like- 

 wife probable, that the furface of water, even when ftag- 

 10 nam, 



