and Spirit Level. 3^7 



them nearly with fpirit of wine, and afcertaining by trial that fide at which the bubble 

 moves moft regularly, by equal inclinations of the inllrument upon a ftage, called the 

 bubble trier, which is provided with a micrometer fcrew, for that purpofe. The moft 

 regular fide is chofen for ^he upper part of the inftrument, the others being of little 

 confequence to its perfeflion. Spirit of wine is ufed becaufe it does not freeze, and 

 is more fluid than water. Ether is better beeaufe ftill more fluid (1). The tube and 

 the bubble muft be of confiderable length. The longer the bubble the more fenfible (2) 

 it is to the fmalleft inclination. A very fmall bubble is fcarcely fenfible, appears as if 

 attached to the glafs and moves but flowly. 



In the ufe of a level of this kind, conftruQed by Sieur Langlois, it was remarked, 

 that when it was properly fet, in the cool of the morning, was no longer fo in the middle 

 of the day, when the weather became hot ; and that when it was again reftified for the 

 middle of the day it became falfe in the evening, after the heat had diminiflied. The 

 bubble was much longer in cold than in hot weather, and when longer it was too much 

 fo, and could not be kept in the middle of the tube ; but flood a little on the one or the 

 other fide, tho' the inclination was precifely the fame. Thefe defefls were fmall, and 

 fuch as claim the notice of careful obfcrvers only ; but they appeared of too much 

 confequence not to produce a wifh to remedy them (3). It was obferved, that they arofe 

 from irregularities in the interior furface of the tube ; and by examining a great number 

 of tubes, felefted for levels of the fame kind, there was reafon to conclude that all 

 thefe levels would have more «|^efs of the fame defefls, becaufe there was not one tube 

 of a regular figure within. They were at beft no otherwife cylindrical than plates of 

 glafs from the glafs-houfe can be laid to be plane before they are grounds The irregula- 

 rities were eafily difcernable. 



It was, therefore, concluded, that it would be advifeable to grind the inner furfaces 

 of the tubes, and give them a regular cylindrical or rather fpindle form, of which the 

 two oppofite fides fhould correfpond with portions of circles of very long radius. To 

 accomplifh this, a rod of iron was taken, of twice the length of the glafs tube, and on 

 the middle of this rod was fixed a flout tube of copper (cuivre) of the fame length as 

 the tube of glafs, and nearly equal in diameter to the bore. The rod was fixed between 

 the centres of a lathe, and the glafs gently rubbed on the copper cylinder, with fine 

 emery and water, caufing it to move through its whole length. The glafs was held by 

 the middle, in order that it might be equally ground, and was from time to time fliifted 

 on its axis, as was alfo the copper cylinder, in order that the wear might be every 

 where alike. The operation had fcarcely commenced, before the tube broke ; and 

 feveral others experienced the fame misfortune, though they had been well annealed. It 



Vol. III.— December 1799. 3F was 



