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[Read before the National Academy of Sciences, November, 1878, at New York, by Prof. 

 George Davidson, A. M., D. Ph., United States Coast Surrey.] 



Geodetic Instruments of Precision at the Paris Exposition 

 and in European Workshops. 



It is a fact familiar to those who have occasion to make use of instrumental 

 contrivances, that novel methods are constantly being devised to meet new 

 requirements or to lessen existing defects. As the delicacy and importance of 

 any given work is increased, the observer soon discovers sources of error that 

 had not been suspected, and finds errors that were veiled by the disposition 

 of the relative parts of the instruments. Instruments of the higher class are 

 too frequently at fault, but the ingenious observer is soon led to investigate the 

 sources of trouble, and either to obviate them or to seek for the best instru- 

 ments suited to his particular and peculiar duties. In the broadest view ot 

 the case we must not be confined to individuals or to nationalities, but aim only 

 to obtain the highest mechanical design and workmanship. In the progress of 

 the geodetic work of the United States, stretching from the Pacific to the At- 

 lantic, and also northward and southward along the Pacific coast, there was 

 furnished to me, among other instruments, one theodolite, of large size, which 

 was subjected to the most rigid scrutiny to determine the flexure of the pillar 

 plate, of the microscope arms, and of the telescope; also, the character of the 

 graduation of the 20-inch circle, and various other points involving accuracy. 

 Under ordinary circumstances the instrument would have passed muster for 

 lines of moderate extent, but when lines of 160 miles in length were involved, 

 it was essential to the integrity of the undertaking, that errors of even a second 

 of arc should be avoided. At the distance of 160 miles one second of arc 

 subtends four feet: a skilled observer can measure a much less quantity than one 

 second, and certainly the instrument should afford the means of his measuring 

 what he can see. 



Fully alive to the importance of the subject, the Superintendent of the U. 

 S. Coast Survey had determined that there should be made a careful examina- 

 tion and study ot the instruments of precision, exhibited at the Paris exhibition 

 for geodetic and astronomical work, topography and hydrography, for the tele- 

 graphic determination of longitude, etc. And, moreover, that the examination 

 should be extended to the first-class workshops of Europe, where instruments 

 of this character are manufactured. 



It was desired to ascertain, if practical, whether the productions of European 

 manufacturers were superior to our own; wherein lay any superiority, if found 

 to exist; and in what consisted the peculiar merit of the work of any particular 

 manufacturer. 



As a mechanician seeking merely the trade secrets of the makers, I could 

 not have expected to enter into any sanctum sanctorum; but upon a candid ex- 

 planation of my purpose and instructions, I found nearly every workshop 



