HYDRODYNAMICS. 



481 



On CapU- if the centre of percussion of all the jets do not coincide 

 'T A l !"Jf~ with the centre of gravity, a rotatory and progressive 

 * motion will be produced. As the jets are generated only 

 on the circumference of the section of the piece of cam- 

 phor, it ought to revolve round an axis perpendicular to 

 the horizon ; and the smallest pieces will obviously 

 turn round with more velocity than larger ones. 



MM. Lichtenberg ami Volta ascribed this rotation to 

 an emanation from the camphor, and also from the ben- 

 zoic and succinic acids, which have the same property. 

 Brugnatelli di>covered, that the bark of aromatic plants 

 when thrown upon water, moved round like camphor ; 

 and Venturi remarked a similar motion in the saw-dust 

 of different woods that had imbibed either a fixed or a 

 volatile oil. Romieu ascribed these motions to electri- 

 city. He found that the camphor sometimes refused to 

 turn, and at other times its movements were suddenly 

 Hopped, by touching the water with particular bodies. 

 The cause of these irregularities, which long perplexed 

 philosophers, was discovered by Venturi. He found, 

 that whenever the water was touched by any body 

 which was tat or oily, or which diffused a small portion 

 of fixed oil, or a great portion of volatile oil over the sur- 

 face, the dissolution and the motion of the camphor were 

 immediately stopped. In order to prove that this ef- 

 fect was not produced by electricity, as Romieu and se- 

 veral Italian philosophers believed, Venturi touched the 

 surface of the water both with conductors and non- con- 

 ductors of electricity, which were well cleared of all 

 oily or greasy matter, and the motions of the camphor 

 were never in the slightest degree affected. When the 

 same substance was afterwards greased with a small 

 drop of oil of olives, and again brought into contact 

 with the water, an oily film immediately advanced over 

 the who'.e surface of the water, repelled the small bits of 

 camphor, and, as if by a magic stroke, deprived them of 

 their apparent vitality. Venturi repeated this experi- 

 ment in a basin of water 20 feet in diameter. The 



camphor turned round in one end of the basin, and 

 when an ounce of oil was poured hi at the other extre- 

 mity, the motions of the camphor were speedily stop. 



IH-.I. 



If the surface of the water on which the camphor 

 srwims, is not sufficiently extensive to allow the 1 

 from the camphor to evaporate, the dissolution of the 

 camphor is either retarded or stops altogether, or the un- 

 disnpatcd liquor forma itself into a thin film upon the 

 surface of the water. In like manner, the particles of 

 the saw-dust of wood soaked in oils, moves quickly when 

 they touch the water ; but their motions do not con- 

 tinue, because the film of oil which they jpmil over 

 the water is not dissipated. 



If the wster is very Mire, and is exposed to a heat 

 even so high as that of boiling water, the i 



and the motions of the camphor are not prevented. On 

 the contrary, they are often promoted by the applica- 

 tion of this heat 



M. \ < nturi applies the preceding principles to the 

 explanation of the motion of the tremelUe observed by 

 Adanson and by Corti. This aquatic plant rise* to the 

 surface of the water during the day, and descends to the 

 bottom at night. If the plant is shut up in a box 

 whose sides are opaque, and if a pencil of light is ad- 

 mitted through an opening in onr of its sides, the tre- 

 melli changes its situation in a few hours, and advances 

 to the hole at which the ligfp penetrates. M. Venturi 

 observed with a microscope, that its branches have a 

 small motion belonging to each of them, in virtue of 

 whi. h they sometime* McObtoAwi one tide to another, 

 vol.. xi. PAKT ii. 



and sometimes advance to free themselves from the Ol > CpU- 

 pencil when they are interlaced. These appearances '"7 Attrac- 



, . , , J , T L i tion and the 



are explained l>v \ cntun on the supposition that the ,.,, 



!!* i ii i i_uncsiui* u* 



water which it absorbs is decomposed by the assistance Kluid*. 

 of light, and that the plant gives out the oxygen in a di- < "Y"^ 

 rection always opposite to the light. Hence it will fol- 

 low that the plant must always move towards the quar- 

 ter from which the light is admitted. 



In addition to the works quoted under the article 

 CAPILLARY ATTRACTION, vol. v. p. 412, the reader is 

 referred to the following: Pascal, Traifes de L'Equi- 

 libre de Liqueur* ft de la retantew de la Matte de L'Air, 

 Avertissement, 2d edit. Paris, 166*. Rohault, Traile 

 t!c Physique, or Dr Clarke's translation of it, under the 

 title of Rohaulti Phi/sica, Lond. 1710, Part I. chap. 

 xxii. 69, 70, 71, 80, 81. Boyle's Ne*> Experiments 

 Pltyuco Mechanical, latching the taring of the air, and 

 ilt 'Effects, made for the most jtart in a fine Pneumati- 

 cal Engine, exp. 35. p. 262. Oxford, 1660. Boyle, 

 Phil. Trant. 1676, vol. xi. p. 775. Hooke's Attempt 

 for the explication of the phenomena observable in an 

 experiment published by the Itight Hon. Robert Boyle, 

 in tie S5/A experiment of hit Epistolical Discourse, 

 latching the air, in confirmation of a former conjecture 

 made by /. : . //. I ...ml.' lf:60. Hooke On Spring*. 4to. 

 1678. Vossius De Nili rt aliorum flummum origin f, 

 Hag. ..bri, Dialog Physici, Lyons, 1669. 



.:, De mottonihnt a firaritale tiaturali pendmlf, 

 I.yotn, 1T>70. Sinclair's Art nova rt magna Gravitatit 

 italis. Rotterdam, 1669- Joh. Christophonis 

 Sturmius, Cnl/egium Experimental* five Curiosum, para 

 i. tentamen viii. p. 44. et auctorium tentam. viii. p. ~~. 

 Norimberg*, 1676. The second part of this work was 

 published at Norhnberg in 1685. .Tamos Bernoulli, 

 Dissrrtatio iir (imitate Elfieris, 1683. De La Hire, 

 Mem. Acad. Par. torn. ix. p. 157. Carro, E.r/x-riences 

 tur let tuymut CmpUlairet in the .A/cm. Acad. Par. 1705, 

 I. Daniel Bernoulli in the Comment. Petropof. 

 177, p. 846. Mariotte Trait edu Monvement det Eaux, 

 vol. ii. p. 105. Par. 170O. Cotes, Hydrottatical I - 

 hires, sect. xi. Lond. 1738. Cigna, Journal ilr /'//- 

 siijuf, torn. iii. p. 109. Lord Charles Cavendish in the 

 Phil. Trans. 1776, p. 383. Monge, Mem, Acad. Par. 

 M. 506. Betue, Journal de Phyiique, vol. xxviii. 

 p. 171 ; xxix. p. 287, 339; xxx.p. 125. Wilson, Edin- 

 mwgk Transactions, vol. iv. p. 144. Venturi, Memoires 

 prfientfs m I.'Institut. torn. i. p. 125. Paris, 1805. 

 MM. Hauy and Tremery in La Place's Snppl. au Dis. 

 Liv. de la Mecanique Celeste. La Place, Supplement 

 a la Theorie dt faction Capillairr, Par. 1 807. Gay Lus- 

 ac, Id. Riot's Traitr de Physique, torn. i. chap. xxii. 

 Paris, 1816. 



CHAP. VI. 

 DESCRIPTION or INSTRUMENT*, AND EXPERIMENTS rox 



ILLt.sTIMTISO THE DOCTRINES Or HYDROSTATICS. 



\.'DcKription of the Mechanic or Hydrostatic Paradox. 



IT appears from Cor. 2. of Prop. iv. p. 428, that the 

 pressure exerted upon the bottoms of vessels filled with 

 fluid, does not depend upon the quantity of fluid which 

 they contain, but solely upon its altitude. This propo- 

 sition has been called the mechanic or hydrostatic pa- 

 rodox, and the instrument for illustrating it has received 



^ f* m . c name - CCCXVH, 



I his instrument is shewn in Fig. 1 . where AB is a box, F%. 1. 



d 



