dnjlninlon and extreme Delicacy of tht Spirit-Level. 135 



Tlie fpirit-level confifts of a tube of glafs nearly filled with the purefl; ardent fpirit, or 

 with highly rectified ether. In order that this iiiftrument may anfwer the intended purpofe, 

 it is neceflary that the inner furface fhould not be cylindrical or ftraight on that fide which 

 is placed uppermoft when in ufe. Suppofe this tube to be laid nearly in the horizontal 

 pofition ; the fpirit will occupy the lower fpace, and the empty part above will exhibit the 

 appearance of a bubble, which will run to the higher end. It follows therefore, fuppofing 

 the tube to be flightly curved, and the convexity uppermoft, that the bubble will be near 

 the middle when the tube is level. In the portable fpirit-level, the tube is properly fet in 

 brafs, and fixed by means of fcrews in a fmall brafs trough, the bottom of which is ground 

 very ftraight. The fcrews are ufeful to place the bubble in fuch a pofition that the lower 

 furface of the trough may be parallel to a tangent foppofed to be applied to the middle 

 point of the curve of the level. The adjuftment is eftefted without much difficulty, by 

 placing the level on an adjuftable plane, and then reverfing it. If the bubble fband accurately 

 in the fame pofition between two marks made on the tube in both fituations of the level, 

 it follows that neither end of the plane nor of the lower furface of the frame of the level 

 is elevated ; or in other words, that every furface to which the level may be applied, 

 and on which the bubble flands in the pofition here mentioned, is horizontal. 



This eafy praxis may be effefted in various ways, according to the nature and figure of 

 the inftrument of which the pofition is to be determined : but the accuracy of the refult will 

 depend upon the fenfibillty of the level ; that is to fay, the fpace pafTed over by the bubble 

 for every minute or fecond of the quadrant, and the certainty with which, under circum- 

 ftances precifely fimilar, it fliall arrive at the fame pofition. In the beft levels the curve 

 rnufl. be circular ; for in fuch the bubble will move with more aftivity, fettle itfelf with 

 more certainty, and defcribe equal fpaces by equal changes of inclination. An ordinary 

 good fpirit level will exhibit a movement of upwards of half an inch for each minute of 

 inclination, and alter the pofition of its bubble by a change of five feconds, or lefs, In 

 fuch a tube the radius of the curve will be about 150 feet. But extraordinary levels are 

 much more delicate. De la Lande * fpeaks of a level filled with ether, the bubble of 

 which pafied over fourteen inches by equal fpaces of one-tenth of an inch for every 

 fecond. The radius of this curve was confequently 17 19 feet, or near one-third of a mile. 

 The tubes of fpirit-levels are fele£led by trial. If a long piece of tube be nearly filled 

 with ardent fpirit, and corked at the ends, the run of the bubble may be tried with a 

 fuitable inftrument called the level-trier, throughout the whole length on all fides. By 

 this means it may be known whether, and in what parts, it may be defirable to divide the 

 tube for the purpofes of filling and clofing. It is remarkable that thefe tubes in general 

 prove either good throughout, or good for nothing ; for it fcldom happens, where one good 

 level can be taken, that the remainder is unferviceable. A refpeftable mathematical in- 

 (Irument-makcr afiures me, that he finds it a good praftice to go to the glafs-houfe and caufe 

 the tubes to be drawn without fufFcring them to be turned round. The ufual praflice in 

 drawing tubes is, th;n two workmen extend a piece of blown glafs, while hot, by drawing 

 the ends afundcr; while a third cools the thinner parts, by blov.ing the airagainil it with a 

 hat. To prevent the glafs from bending downwards by its own weight, the workmen are in 



** In hij Aftronomic ; no: new »t hand. I fpcak from mcmary ; but have no Joubt of the numbers, nor 

 of the hfX. 



6 the 



