330 Mr. J. Nixon on the Measurement of the 



and the depression of the same as seen by reflection, were 

 measured in succession, first on one arc, and, immediately 

 after inverting the cylinder, also on the other. The reversing 

 points of the levels were well ascertained; the stand of the 

 sector being previously glued to the plank, which was kept at 

 one constant inclination b} T referring to Fortin's level. The 

 measurements, of which the details follow, give on an average 

 26" for the error. 



January 24th, 1833. Temp. 48° Fahr. 



Means. 



Depression of ditto reflected .{g$gg*« *j $} = "* 



Difference 50£ 



Half difference, or constant error 25£ 



Elevation of .ower li m b of dise { ^ g^«^ g g }* 53 21 



Depression of ditto reflected (g #£«£ «j jP) = 54 15 



Difference 54 



Half difference, or constant error 27 



Ditto by upper limb 25J 



Mean ... 26J* 



By the Seventh Method. 



Theory. — Let ES (fig. 2.) represent the elevated line of 

 collimation of a telescope ; L the axis of a perfectly cylindrical 

 level-tube, moveable about a horizontal pivot fixed to any part 

 of the telescope. The axis L, as the bubble is supposed to be 



* Remarks. — 1°. When the horizontal wire of the sector appeared on a 

 level with the upper limb of the reflected disc, the height of the fluid in 

 the cistern was increased nearly 0-2 inch, without diminishing in the slight- 

 est degree the depression of the image. 2°. A better image was obtained 

 from water resting on quicksilver (the light being derived from white paste- 

 board) than from either of those fluids, or from treacle, alone. 3°. In every 

 case the image appeared a little more distinct when a sheet of white paper 

 extended horizontally about a tenth of an inch above the surface of the 

 fluid. 4°. However modified the light or apparatus, the reflection of a hori- 

 zontal wire could not be rendered visible. 5°. The image of the vertical pearl 

 slip was not seen, perhaps from its length, except at a great depression of 

 the collimator; but were the aperture of the stop considerably contracted, 

 the line drawn across the diaphanous slip might be observed with greater 

 certainty than either limb of the minute disc. 



