90 Mr. Nixon on the Measurement of the 



iron (?) surfaces (used in grinding the receivers of air-pumps), 

 fixed pai'allel to the horizon and level with each other by 

 means of an adjusted spirit-level. 



A few trials with the proof-level indicated that the cylin- 

 drical ring of the sector the nearest to the object-glass was 

 slightly conical; for although the proof-level had but little 

 play between the shoulders of the two rings, yet the nearer it 

 was moved towards the eye-end shoulder the greater the de- 

 viation of the bubble in the opposite direction ; the total range 

 being about four seconds. 



To obviate this cause of inaccuracy, a long smooth hone 

 having an even surface was fixed upon a table, and at one 

 end of the hone was secured, in a line with it, a board rising 

 some little above the level of the hone. Hence by drawing 

 the proof-level to and fro, and always in one direction, with 

 one plate on the well-oiled hone* and the other on the board, 

 the outer end of each plate was ground down at one and the 

 same angle. To reduce the inward ends of the plates the 



hone w-as equally raised above the . 



level of the board. The plates were V~ j 



now of the figure in the margin, so ^ ,0^-^ 



much only of the original surface 



being left as would come in contact with the cylindrical rings 



of the sector exactly over their Ys. 



To each side of the frame, at an equal distance from the end 

 of it, was attached an upright piece of wood, both pieces being 

 notched above for the reception of a level-tube furnished with 

 a divided scale of about J^ th of an inch to 2". This cross 

 level, which served to set the brass plates transversely level, 

 was adjusted in the following manner. A brass bar, 20 inches 

 long, 4 inches broad, and | an inch thick (including the 

 rim), professedly ground to an even surface, was placed upon 

 the stone pillar and secured at the corners by four heavy 

 weights. The bar being of uniform thickness and the top of 

 the pillar nearly horizontal, the proof-level could be placed 

 with its ground plates in contact with the upper surface of 

 the bar and reversed in direction without displacing the bubble 

 of the cross level beyond the limits of its scale. Hence the 

 plates would be horizontal in a transverse direction when the 

 middle of the bubble stood at a point of the scale equal to the 

 mean of the readings before and after reversing the frame. 



The brass bar, although certainly not perfectly even, could 



* To prevent the plates being burred in the operation the hone was 

 constantly supplied with oil, and no other force applied than the weight of 

 the bar. 



not 



