102 Mr. J. Nixon on the Measiivcmetit of the 



As an indispensable verification, the two proof telescopes 

 were placed, at the conclusion of the measurements, once more 

 together on the plank with their bubbles at their original marks, 

 when their lines of collimation were found to be parallel, to the 

 accuracy of about a second. 



A preliminary trial, unsatisfactory from some uncertainty 

 respecting the reversing point of one of the great levels, made 

 the error 20". The mean of three succeeding measurements, 

 of which the particulars are subjoined, gives 22"*8, or, if we 

 reject the first set, 22"-3, for the instrumental error. (The dis- 

 crepancy between the results by the two levels is a conse- 

 quence of the line of collimation of the sector not being strictly 

 parallel to the axis of the cylinder). 



March 19tb, 1831. Temp. 51° Fahr. 

 Depression of (the line'i ^ 



of collimation of) the > by right-hand level 27"0 Error. 



round telescope ) ,, 



by left 26-8 >•= 23-9 



Depression of the square "I i • i . no r 



S ' >bv right 22*5 



telescope j •' ° 



by left 19-4_ 



March 21st, 1831. Temp. 57° Fahr. 



Depression of the round ^ , • i ^ i i i i ^)^ i "1 

 S > bv i'i2;ht-hand level 26*1 



telescope j '' *- 



by left 28-5 I .,2-6 



Depression of the square \,^..^j^^ j^.^ j 



telescope J '' " 



by left 20-5J 



March 21st, 1831. Temp. 60° Fahr. 



Depression of the round |^^ right-hand level 24-5] 



telescope J -^ ° I 



by left 27-7 I 22-0 



Depression of the square K.j^jjj. ^g.^ 



telescope j '' " 



by left 20-5_ 



To the above measurements may be added a single set 

 made with the sector placed first on the plank, and afterwards 

 raised by introducing a half-inch board between it and the 

 plank. For want of leisure, the parallelism of the proof tele- 

 scopes was not verified at the close of the operations ; but as it 

 was examined in the interval between the two measurements, 

 no sensible deviation could have possibly taken place in the 

 brief interim. The calculation for the sector on the board 

 comes out 1" less than with it on the plank, but the sector 



