ggS ConJiruBion of the Spirit Levd. 



was fuppofed that the emery which became fixed in the copper* might contribute to 

 fplit the glafs, each grain continuing its iinprefTion with the fame point, in the fame 

 right line, which in feme inftances might be as well difpofed to cut the glafs aafa diamond. 

 A cylinder of glafs was fubftituted, inftead of the copper, and the emery rolling itfeU" 

 oil the furface of the laft, inftead of fixing itfetf, had better fuccefs ; fo that ever)' part 

 of the circumference of the tube and the cylinder touched each other through their 

 whole length. The fame operation was continued, ufing finer and fiiier emery to fmooth 

 the tube, and prepare it for polidiing ; after whicb the tube and cylinder having been 

 well wafhed, thin paper was pafted round the cylinder, and the paper was very equally 

 covered with a fmall quantity 'of Venice Tripoli. The tube was then replaced and 

 rubbed as before, till it had acquired a polifht. 



A level thus ground, may be either of the proper fenfibility, or be too much or too 

 little fenfible. It will be too (luggifh, if before grinding, exclufive of the irregularities 

 of the tube, its diameter fhould much exceed in the middle of the length the diameter of 

 the extremities ; or it will be too fenfible if this diameter fhould not fufficiently exceed the 

 other ; or laftly; if the middle diameter be fmaller than that of the extremes, the bubble 

 will be incapable of continuing in the middle, but will, in every cafe, either run to one 

 or the other end, or be divided into two parts. 



To correft thefe defefls, and to give the inftrument the required degree of perfeflion, 

 it is proper to examine its figure before the grinding is entirely finifhed. For this pur- 

 pofe, after cleaning it well, a fuflScient quantity of fpJM of wine muft be put into it ; 

 and fecured by a cork, at each end. The tube muft then be placed on the forks or Y's 

 of a bubble trier, and its fenfibility, or the magnitude and regularity of the fpace run 

 over by the bubble by equal changes of the micrometer fcrew, muft be afcertained. If 

 the run or fpaces pafled over be too great [g] they may be rendered fmaller by grinding 

 the tube on a ftaorter cylinder ; but if they be too ftiort, they may, oir the contrary, be 

 enlarged, by grinding on a longer cylinder. It is neceftary, therefore, to be provided 

 with a number of thefe cylinders of the fame diameter, but of diflJercnt lengths, which 

 it is advifeable to bring to a firft figure, by grinding them in a hollow half cylinder of 

 brafs. By means of thefe it will be eafy to regulate the tube of the level to any required 

 degree of fenfibility, after which the tube may be very quickly fmoothed and poliflied. 



* 1 am informed that our artifts ufe brafs (cuivre jaune) without this inconvenience. It is probable that the 

 «oppcr being defended by the emery bedded in its face did not itfelf grind away fo as to fit the glafs during the 

 operation, and therefore broke it. This property of copper is well known to glafs cutters and other artifts. A 

 drill, confifling of a very thin copper tube will, with emery, cut a hole through glafs very readily ; and a fmall 

 circular plate of the fame metal, no thicker than a card, will cut through a piece of glafs or the hardeft file, 

 without any perceptible wear of the copper itfelf. — N. 



f Our artifis find the poliibing rather noxioits than beneficial, and have, therefore, difcontinued it. — N. 



' The 



