2 THE CORRECTION OF SEXTANTS FOR ERRORS 



now quite exceptional, and there is apparently no reason to anticipate a 

 reversal of this verdict. It is very desirable, therefore,' to possess some 

 efficient means of correcting the errors which are peculiar to the sextant. 



The eccentric corrections of a sextant may be deduced from measure- 

 ments made with it of three or more known angular distances. A trust- 

 worthy determination requires several angles, of such magnitudes that 

 no part of the arc shall be very far from one of the readings obtained. 

 The known angles may be the apparent distances between stars, computed 

 from the positions given in the catalogues, or the angles subtended by 

 well-marked terrestrial objects, measured by a theodolite or otherwise. 

 One of these methods will usually suffice for an observer who wishes to 

 obtain corrections for his own instrument, but neither of them can be 

 regarded as generally available when large numbers of sextants are to 

 be examined, for besides the laborious computation involved, stars are 

 seen only at night and in clear weather, while a spot where suitable 

 terrestrial objects at proper angles and sufficient distances are always 

 visible is sometimes difficult to find. 



During the latter part of the civil war the writer, then an assistant in 

 the U. S. Naval Observatory, was required to superintend the repairs of 

 sextants returned in unserviceable condition from the various fleets. 

 Some of these instruments bore marks of violence, and it was felt to be 

 extremely desirable that none of them should be reissued until their 

 integrity had been verified by some adequate test. A plan for making 

 such tests was accordingly formed, but circumstances did not permit it 

 to be carried out at that time. Not long afterward, however, while 

 engaged in a similar occupation at the Hydrographic Office, the apparatus 

 about to be described was constructed. In the meantime a description 

 was published* of Mr. T. Cooke's apparatus, which had recently been 

 set up at the Kew Observatory for the same purpose. Disregarding 

 minor details, the apparatus at Kew consists essentially of a series of 

 horizontal collimators equidistant from, and directed toward, a central 

 point where the sextant under examination is supported in a horizontal 

 position by a small table. The collimators are firmly secured to a curved 

 pier of substantial masonry ; the angles which their axes make with each 

 other are measured by a theodolite temporarily placed on the central 

 table for that purpose, and are presumed to remain nearly constant. A 

 reraeasurement of these angles with any sextant affi)rds a comparison 

 between its graduation and that of the theodolite, at points depending 

 upon the number and disposition of the collimators, which at Kew indi- 

 cate five axial directions, including angles approximating 15°, 30°, 45°, 

 60°, 75°, 90°, 105°, and 120°. 



* Balfour Stewart, Proc. Roy. Soc, London, 1868, xvi, 2. 



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