230 Mr Galbraith on a New Pocket-Box Circle. 



will be obtained, giving a probable error not exceeding 0" or 

 so, for each double observation. By repeating these two or 

 three times when thought necessary, it is obvious a much greater 

 degree of accuracy will be obtained, than, from the small size 

 of the instrument, might naturally be expected. Its conve- 

 nient form, the ease with which it may be used, and the accu- 

 racy of its results, will, it is hoped, recommend it to a nume- 

 rous class of observers, while its moderate price will enable 

 many to become purchasers. 



My remarks relative to the advantages derived from the use 

 of two or three microscopes or verniers, are derived chiefly from 

 an excellent analysis of the errors to which astronomical circles 

 are liable, in a paper by Dr Robinson of Armagh, read before 

 the Royal Irish Academy in 1825, and, if I am not mistaken, 

 published subsequently in a separate pamphlet, so that it ap- 

 peared to me unnecessary now to enter upon another discussion 

 on this subject. 



The learned author shews that " two microscopes will correct 

 for eccentricity, or for any other error varying by a similar 

 law ; as also for all errors which are as the odd powers of the 

 sine or cosine, but that three verniers are still better, failing 

 only where the number expressing the order of the error is di- 

 visible by three." 



In the original circle exhibited to the Society of Arts for 

 Scotland, there are three verniers, but in the figure given here 

 there are only two, though either mode may be followed by the 

 maker when desired. In this figure A B C D is the circle di- 

 vided into 720, A the telescope, B, D, the two opposite ver- 

 niers reading to minutes, or half minutes, if thought neces- 

 sary, K the reading microscope or lens, G I the tangent screw 

 fixed at any required position on the elevated circle E F H by 

 a clamping screw near H. From this description, the method 

 of making and using the circle will readily occur to those ac- 

 quainted with the sextant. 

 54. SOUTH BRIDGE. WILLIAM GALBRAITH. 



