Instrumental Eiror of his Horizon-Sector. 347 



two substantial blocks of wood were glued, nine inches asun- 

 der, to the upper surface of the plank towards its northern 

 end; (occupying the place of the lantern as represented in the 

 figure, p. 338.) These blocks, notched in the middle in a line 

 with the cylinder and telescope, supported, on a level with the 

 two latter, a tube of wood It inches long and 1'5 inch in dia- 

 meter, containing within its southern end the additional ob- 

 ject-glass and tube, and a fine eye-piece (by DoUond) within 

 the northern end. Distinct vision of the cross lines, ^xed at 

 right angles to each other, between the first and second glasses 

 of the eye-tube, being obtained by regulating their distance 

 from the moveable eye-glass, the object-glass was adjusted to 

 the proper focus by pointing the tube (telescope) at the sails 

 of a distant windmill. 



Method of Observation. — The cylinder being removed, its 

 place within the Ys was supplied by a tin tube (of the same 

 diameter) blackened within, which reached so nearly from the 

 object-glass of one telescope to that of the other that all false 

 light could be excluded by a large piece of black cloth cover- 

 ing the tube and object-glasses. Having placed the lantern 

 a little beyond the eye-glass of the round telescope, the inter- 

 secting point of the cross threads of the square telescope were 

 moved by their micrometer screw until they bisected the ho- 

 rizontal wire of the round telescope where crossed by the ver- 

 tical one. Removing the lantern to beyond the eye-glass of 

 the square telescope, the bisection, on looking through the 

 round telescope, did not appear quite perfect or certain, being 

 slightly affected by evident parallax. 



Lastly; the cylinder containing its proper object-glass only, 

 being substituted for the removed tin tube, the line of colli- 

 mation of its object-glass, which was situated close to that of 

 the round telescope, was made parallel by the rack-work of 

 the stand to that of the latter; or rather, as nearly so as the 

 almost impossibility of placing one wire exactly before another 

 would admit of. The cylinder being reversed, the great levels 

 were read offi before, and then again after the line of coUima- 

 tion of its object-glass had been made parallel, by the rack- 

 work of the stand, to that of the square telescope. In making 

 the bisections, the lamp stood on separate apparatus near 

 the eye-glass of the opposite telescope. 



The results (scarcely worth transcribing) varied in eighteen 

 measurements from 1 5"-5 to 27"-5 ; mean 21"-3, or 8" more 

 than by the Eleventh method. 



John Nixon. 

 [To be conlinucd.J 



2 Y 2 XLIII. Notice. 



