OP A NEW STATICAL LAMP. 9j 



occupy the fpace between the two ends fg and de, and the Defection of a 

 oil by preffing on the furface of the mercury at rr, will raife ™" ftat,wl 

 it a little on the outfide from its height and fpecific gravity. 

 The fpeciric gravity of oil to that of mercury is about as one 

 to 16, a column of oi-1 therefore of 16 inches would not raife 

 the mercury above an inch *. 



If, after having filled the refervoir with oil, and fcrewed on 

 the focket, the Plate fg be fo adjufted with weights, that it 

 (hall become equal in weight to a column of oil whofe bafe is 

 fg, and height ks ; it is evident that the oil in the refervoir 

 will under this preflure rife to s s, and will conftantly remain 

 at that height while any remains, and while the weight of fg 

 continues unaltered, notwithstanding the fall of fg, which 

 when the oil is confumed, will be deprefled to de, and refl 

 on the Plate de. uutt, is a fmall tin tube, intended to re- 

 ceive the oil which may be fpilled ; it anfwers the* purpofe 

 of the fmall round glafs ufually hung beneath other lamps. 



The height to which the oil may be raifed in a lamp of this 

 kind is arbitrary, fince to increafe the height of the focket it 

 is only neceflary to increafe the weight in proportion, always 

 providing a fpace for the rife of the mercury, equal to a 16th 

 part of the rife of the oil. This rife will be conftant, the 

 weight being always the fame/ as it does not in any refpeel 

 depend on the quantity of oil in the refervoir. 



It is neceflary, however, to obferve refpe6ting the inva- 

 riability of the counterpoifing weight, that it will not be geo- 

 metrically accurate. We know by the laws of hydroftatics, 

 that a folid body lofes by immerfion in any fluid as much of its 

 weight as is equal to the weight of the fluid difplaced by it; 

 now as the oil is confumed, the middle veflel finks deeper into 

 the mercury, and confequently lofes of its weight, it will 

 therefore after fome time be evidently incapable of fupporting 

 the column of oil at the fame height as before. But if this 

 middle veflel be made of very th'in fheet iron and has not much 

 range, this diminution will be inconfiderable, and may in prac- 

 tice be entirely neglected. The lamp in the figure being ad- 

 jufted to 10 lines, will contain oil fufficient for eight hours, at 

 the end of this time, the furface of the oil in the focket will 



* L e. Half an inch rife without and half an inch depreflion 

 within. N. 



have 



