ADVERTISEMENT. 



Improvement in the construction of instruments of precision is neces- 

 sarily preceded by the development of means for detecting the errors of 

 those already in use. The sextant, though primarily an instrument 

 for the traveller, will, when carefully made and handled, give results of a 

 remarkable degree of precision ; and it is a matter of great importance 

 that even for the ordinary purposes of navigation every precaution should 

 be taken to free it from all classes of instrumental error. Inaccuracies of 

 mechanical construction, though very miuute from the artisan's standard, 

 are greater in effect than even experienced navigators sometimes realize. 



An apparatus for investigating and determining such errors of the sex- 

 tant, devised by Mr. Rogers (the theory and use of which is described in 

 the present treatise), has been extensively employed by the officers of the 

 United States Navy in testing sextants before their issue to the ships of 

 the naval service. 



It may be added that, in the constant re-action between advances in 

 science and art, the reciprocal benefits of which should always be freely ac- 

 knowledged, the skill of the instrument-maker Tespecially in the accuracy 

 of graduation of circular measures) is still behind the delicate requirements 

 of modern investigation ; and it is confidently believed that methods of de- 

 tecting minute inaccuracies will result, as in the past, in stimulating the 

 artisan to further refinements in instrumental construction. 



S. P. Langley, 

 Secretary of Smithsonian Institution. 



"Washington, December, 1890. 



