Mr. Nixon oti the Curvature of Sin rit-Levels, 177 



one of the levels of the sector, the variation of inclination be- 

 ing the same for both levels, it appeared that one division of 

 the scale of the former, also of forty to the inch, was equal to 

 1"-4S0*. (In this and the following experiments, the axis of 

 the short level was invariably made parallel to that of the 

 sector level, the line on \yhich the divisions are drawn being 

 perpendicular in the two levels to their respective axes. As 

 neither of the levels might be uniformly curved throughout, 

 they were rendered strictly parallel in horizontal inclination ; 

 and care was taken never to drive the bubbles completely to 

 the extremities of the tubes. The values quoted are the mean 

 of three or four satisfactory measurements of about 100" each. 



No. II. — The Ys being thinly coated with glue, the level was 

 placed gently within them in the preceding position, and suf- 

 fered to dry without pressure. The divisions were then found 

 to be l"-683 each. 



No. III.— The level was placed on the even surface of one 

 of the brass indices of the sector previously spread over with 

 glue, and left to dry. The divisions were now l"'679 each. 



No. IV. — The level being glued to the surface of a maho- 

 gany bar, eight inches long by one inch square, fixed to the 

 cylinder of the sector, the divisions, after a lapse of a couple 

 of days, were found to be l"-854 each. On repeating the mea- 

 surement on the following day, the value appeared to be 

 l"-853. 



No. v.— The level was mounted in a brass case with a de- 

 gree of tightness barely sufficient to preserve it unvaried in 

 position. — Value of one division = l"-684. 



No. VI. — The level being loose widiin the case, the packino- 

 (wiUi cotton wool) was confined to the ends of the tube.— Value 

 of one division = 1"-915. It must be supposed that the mere 

 pressure of the packing had distorted the tube ; otherwise, 

 why did the curvature differ from its value in the first expe- 

 riment ? 



No. VII.— The level was placed, unattached, within the ma- 

 hogany Ys, overhanging two inches at the smaller end of the 

 tube, and scarcely half an inch at the opposite end.— Value of 

 one division = 1"'615. 



No. VIII. — The Ys being set up more distant from each 

 other, the level, i)laced widiin them unattached, extended at 

 each end no more dian a (juarter of an inch beyond the Ys. 

 — Vaiueof one division = l"-703. The mere weight of the level, 

 It would appear, had not diminished the degree of curvature. 



1*1 ^ I'a."''^''" ?^ '""'■ P''ccecling measiircnients, rejected on account of a 

 Elight difference in the degree of inclination of the two levels, jravc 1"-17!) 

 for the v;iliie of one ilivision. 



N. S. \'ol. 5. No. 27. Mcnr/i 1H29. 2 A Al- 



