ON HYDROSTATIC INSTRUMENTS, &c. 237 



traded by the addition of another substance : thus iron, by the addition of 

 erne eighth of its bulk of platina, becomes contracted one fortieth of that 

 bulk. 



The use of the spirit level depends on the tendency of all fluids to pre- 

 serve a horizontal surface, and the freedom with which the particles of 

 fluids move on each other, renders it an instrument capable of the greatest 

 delicacy. A tube which is very slightly curved, being nearly filled with 

 alcohol or ether, and then perfectly closed, the bubble will always rise to 

 the highest part of the tube, and will never be stationary at the point which 

 is marked as its proper place, unless the instrument be very accurately 

 horizontal, or in the same position in which the mark was adjusted. The 

 surface of the bubble, especially when it is small, cannot, in a strict sense, 

 be called perfectly horizontal, since its form approaches nearly to that of a 

 sphere ; but in order that the centre of gravity of the water may attain the 

 lowest possible situation, the bubble must necessarily occupy the highest 

 point of the tube. (Plate XXI. Fig. 279.) 



The principles of hydrostatics have been employed in various ways for 

 supplying lamps with oil. It is found that a lamp will burn, without con- 

 suming any considerable portion of its wick, as long as it is amply supplied 

 with oil ; hence it becomes desirable that it should always be level with 

 the surface of the reservoir, and this may be effected sufficiently well by 

 placing the wick at the edge of a very large vessel, or at the end of a tube 

 projecting from such a vessel, or from a vessel closed above, and opening 

 only by an orifice below, which lets in the air as the oil escapes through it. 

 But all these methods are often attended with inconveniences of various 

 kinds, especially where the lamp is to be employed like a candle, and 

 placed on a table. A French artist has applied a little pump, which is 

 worked by means of a spring, for raising the oil from a vessel under the 

 lamp ; but this refinement is too complicated to be practically useful. Mr. 

 Keir's lamp * contains a divided cavity, one part of which is filled with oil, 

 and the other with a saline or saccharine fluid of greater density, so that 

 when the oil contained in the upper part of the tube is exhausted, its place 

 is partly supplied by a fresh portion, which is forced up in consequence of 

 the descent of the denser fluid in a much larger vessel. Still, however, the 

 surface must be lowered by degrees ; but by combining the invention with 

 Dr. Hooke's semicylindrical counterpoise,t a little modified, the height of 

 this fluid may be so regulated, that the surface of the oil may remain 

 almost invariable, until the reservoir is quite exhausted. For this purpose, 

 the centre of gravity of the counterpoise must be a little higher than the 

 line which bisects it ; and its specific gravity must be about three fourths 

 as great as that of the fluid ; and in this manner it may be made to raise 

 the surface of the heavier fluid, in proportion as a greater quantity of it 

 escapes, to supply the place of the oil ; and to keep it always at a sufficient 

 height above the surface which separates it from the oil, so that the wick 

 may be amply and almost uniformly supplied. (Plate XXI. Fig. 280.) 



The art of embankment is a branch of architecture entirely dependent on 



* Nich. Jour. iii. 467. t Lampas, p. 188. 



