Piiaciple of Motion in a jioating Lamp. l6g 



-tiguous to it. This became manifeft when very fine charcoal dull was lightly fcattered 

 around the lamp. As it then prcceeded in its courfe, it marked out a fpreading or diverg- 

 ing vjake behind it entirely clear of all dull, in coniequence of the particles being chafed 

 backwards and laterally with a motion mucn more than merely relative. 



Defirous of learning how this difpofition ot the duft would take phce when the lamp 

 was ftationary, I conftrnfted one of a fine wafer, and with an excentric wick confifting of 

 a foft cotton thread doubled ; and to prevent the wafer or baL' from catching fire I coated 

 its upper furface with gold leaf. When this was made to reft immoveably upon the oil, the 

 dufl retired in all direftions fo as to leave the fpace adj.-.cent to the wafer quite free from 

 every particle. But here it was obfervable, that this difperfion of the duft, by the feeming re- 

 pulfionof the bafe of the lamp, was much more rapid at that fide which lay nearefl to the 

 wick than at any other part, and lead of all fenfible at the fide diametrically oppofite. 



Tiie circumftances laft menticned feem fufficiently t > account both for the progrelTive 

 motion of the lamp, and for the general law of this motion formerly defcribed. For, re- 

 garding this difperhon of the duft as yet only in a general way, and as theeffc£l of fome re- 

 pulfion between the bafe and the oil contiguous to it, the fadts above -inentioned plainly in- 

 dicate, that in ail cafes this repulfion is ftrongeft at that part of the bafe neareft the wick or 

 flame ; and as a£lion and readtion are equal and contrary, the lamp muft theiefore be im- 

 pelled, in the direflion of a line drawn through the wick, towards that part of the bafe moft 

 remote from it, and where the readtion is the leaft. 



But in order to obtain a ftill more competent knowledge of the phyfical catife of thefe mo- 

 tions, it feemed now neceflary to enquire more particularly into this apparent repulfion, 

 between the bafe of the lamp and the furrounding oil, as indicated by the difperfion of the 

 <3uft in the manner above defcribed : and here the following confiderations prefented them- 

 felves : 



/ rThe oil in the bafon, when of an uniform temperature, has all its parts in a ftate of equi- 

 librium and of reft. When the lamp is lighted, it is evident we have a very aflive caufe in- 

 troduced tending to deftroy that equilibrium. This caufe is the flame which broods over 

 a fmall portion of the oil, and is feparated from it only by the intervention of a piece of 

 paper or a wafer. The oil in fiich circumftances, in confequence of being violently heated, 

 muft fuddenly increafe in volume, and muft now, on account of the decreafe of its fpi-cific 

 gravity, be prefTed upwards by a force fufficient to raife part of it above the general level. 

 But this heated portion of oil, in its endeavour to rife up, will meet with a refiftance equal to 

 the weight of the incumbent lamp, which will determine it, in feeking a vent, to Aide out 

 from under the bafe in a thin fuperficial ftream ; and it feems to follow with equal certainty, 

 that this conftant ftream will flow moft rapidly and moft copioufly towards that fide of the 

 bafe of the lamp where the refiftance is leaft, or where it has the fhorteft way to prefs for- 

 ward ; that is, from under the wick or flame to the edge of the bafe which is the neareft, 

 according to what we have f^en to be agreeable to the phenomena. But, from the laws of 

 motion, it is certain that the re-adlion of this ftream of rarefied oil, thus ilTuing moft rapidly 

 and moft copioufly from a particular fide of the bafe, muft impel the lamp in the contrary di- 

 re£\:ion,and make it fail in the manner we have feen. It may further be remarked, that the 

 heated oil fo retreating from the flame, and endeavouring to rife fomewhat above the general 

 level, in confequence of its diminifticd fpecific gravity may more or lefs lift up that fide 

 Vol. II. — July 1798. Z of 



