468 Defcription of an Hydrojlatic Lamp. 



cepted as little as poffible. It happens unfortunately that thefe two conditions have ufu- 

 ally been found to militate againft each other. If the receptacle for oil be made tall and 

 thin, the light will indeed be difperfed like that of a candle through a very large portion 

 of the furrounding fpace or fphere ; but as the confumption of a fmall portion of oil will 

 oocafion a confiderable fall in fo narrow a veflel, the fluid will foon be deprelTed too low 

 for the capillary attra£lion of the wick to raife it with cfFe£t, and confequently the light 

 will firft decay and then go out. If on the other hand, the oil veflel be made broad and 

 comparatively fliallow, the flame will certainly be maintained for a long time, becaufe a 

 large quantity of oil muft be confumed before the fall will be of any pradlical confequence ; 

 but at the fame time the fliadow of this broad veflel will obfcure nearly an entire hemi- 

 fphere, if the wick be in the middle of its upper furface, and it will obft;ru(St much of the 

 light even in the moft favorable pofuion of the burner, namely, on one fide or edge of the 

 veflTel. 



To remove thefe inconveniences as much as pofTible, there have been a variety of con- 

 trivances, among which the inverted veflel called the Fountaia, is the moft remarkable 

 and the moft efteemed. This veflel, or receptacle of oil, may be confidered as a bottle 

 filled and Inverted with its neck plunged in a bafon of the fame fluid. The experiment 

 may be familiarly made with a bafon of water, to the fatisfaftion of ihofc who are unac- 

 quainted with the efletts of the preflure of the atmofphere, which prevents the water from 

 quitting the bottle. In this fituation it is feen, that when the water in the bafon is by any 

 means depreflTed, a bubble or portion of ait forces itfelf into the bottle and rifes to the top, 

 while an equal bulk of water defcends into the bafon. Or the fame effeft may be feen in 

 a glafs apparatus very common in bird cages. In this manner it is that the oil flows down 

 and fupplies the confumption in lamps, which are provided with a refervoir of this kind. 



The ftiadow caft by the vafe or refervoir of a fountain lamp, is an inconvenience which 

 may be much diminiflied by fixing the burner at the end of a tube or branch made as long 

 as may be convenient. Another inconvenience is however lefs fufceptible of an adequate 

 remedy ; namely, the expanfion of the air included in the upper part of the refervoir, " 

 when the temperature of the place is by any means raifed. This expanfion will caufe the 

 oil to defcend in greater quantity than the combuftion demands, and has been found very 

 prejudicial in aflembly rooms, and other places of general refort, by the overflow of that 

 fluid upon the floors, as well as the clothes of the company. It feems as if the famous 

 Robert Hooke, who could not have been unacquainted with the fountain lamp, was led 

 to the invention of his very ingenious lamp with a float, from reflections on this imper- 

 fe£iion. This lamp, which is defcribed in Birch's.Hiftory of the Royal Society, confifted 

 (Of an hemifpherical veflel to hold oil, with a lip at one edge for the wick. An hemifphe- 

 rical folid was conftrufted fo as very nearly to fit the cavity of the veflel, and was fuf- 

 pended by an horizontal axis, upon which it could freely move or librate. It would there- 

 fore hang in the veflel and almoft totally fill it, when not occupied by any other fubftance ; 

 but as its fpeciiic gravity was defignedly made equal to half the fpecific gravity of oil, it 



would 

 3 



