228 Mr Galbraith on a New Pocket Box-Circle. 



Mayer, contained in his Tabulae Solis et Lunae, published at 

 London in 1770. This was the first reflecting 1 circle to which 

 the repeating principle was applied. It was also the opinion 

 of the inventor that a circle could not be divided at that time 

 to a greater accuracy than three minutes, the dividing engine 

 not being then in existence, and for these reasons sixteen inches 

 was fixed upon as the diameter of this circle, which Bird, in 

 the actual construction, exceeded by an inch and a half. The 

 consequence was, that the circle became far too heavy to be 

 used in the hand, either at sea or land. 



This circle was divided as usual into 360, and therefore it 

 was necessary to double the angle read off from the instrument, 

 as required by the principles of optics, on account of the double 

 reflection at the index and horizon glasses. To avoid the 

 trouble of doubling, these instruments have, for many years, 

 been divided into 720, or the space on the arc equal really to 

 half a degree, is now reckoned a whole degree. 



Admiral Campbell having found the instrument, on account 

 of its weight, unmanageable at sea as a repeating instrument, 

 confined his observations to a part of the limb that was found 

 to be most perfectly divided. This gave rise to the construc- 

 tion of the sextant, which was lighter and more easily managed. 

 These advantages, however, were gained at the expense of su- 

 perior accuracy. From the principles of construction, a circle 

 with three verniers has the errors of centering, division, read- 

 ing, glasses, &c, all either exactly, or nearly corrected ; while 

 in the sextant the accuracy depends almost entirely upon the 

 abilities of the artist. Now, we think it cannot be doubted 

 that the principle of correcting errors by mechanical means is 

 much to be preferred to trusting to the character and abilities 

 of the artist, however excellent both may be. For this purpose, 

 even Troughton himself, with all his skill and ability as an ar- 

 tist, contrived a somewhat large reflecting circle of about eight 

 or ten inches in diameter, and the same thing has been done by 

 Dollond, Borda, Mendoza Rios, Hassler, &c. For many pur- 

 poses these are too large and heavy, especially for travellers, 

 surveyors, &c. who have generally had recourse to the pocket- 

 box sextant, which is subject to the same errors as the larger 

 sextants already alluded to ; and to avoid these, I, many years 



