308 



SCIENCE. 



[N. S. Vol. IX. No. 218. 



FKOM FIGUEE ADJUSTSIENT. 



The largest correotion is 0".84 



And 55 per cent, of the corrections are less than . . .25 



The probable error of the side Ibepah- 

 Nebo, depending on angular measures only, 

 is 1/280,000 of its length. 



Heliotropes were continually employed, 

 and the angles were measured with a the- 

 odolite having a horizontal circle of 20" 

 diameter and a magnifying power of 83. 



ASTEONOMICAL WORK. 



Aside from the work in practical astron- 

 omy incident and necessary to the opera- 

 tions of every trigonometrical survey, at- 

 tention has been given to various other 

 phases of the subject. It has not alone 

 sufficed to point out and demonstrate the 

 utility of the method of equal zenith dis- 

 tances for latitude, and of the application of 

 the telegraph to longitudes. The Coast and 

 Geodetic Survey feeling the necessity of 

 better star places, arising from the use of the 

 methods just mentioned, has devoted some 

 of its energy to the perfection of star cata- 

 logues. It is probably no exageration to 

 say that the declinations given in our field 

 lists are the best attainable anywhere. 

 More than fifty of the best modern cata- 

 logues are corrected for their systematic 

 errors, and each is given weight depending 

 on the value of the work and number of 

 observations. A collection of all these data, 

 and their consolidation into one homoge- 

 neous result, eliminates as far as possible all 

 known sources of inaccuracy, and gives us 

 finally the most reliable positions. A list so 

 constructed of several thousand stars has 

 been already published, many of which are 

 especially adapted to southern work. The 

 average probable error of a declination may 

 be given as rather less than ^ of a second ; 

 a degree of precision, which enables an ob- 

 server to determine his latitude from 20 

 pairs, in one evening, with an uncertainty 

 of only ±10 feet. This is sufficient for the 



purposes of geodesy. Incidental to regular 

 astronomic work, the Survey has equipped 

 and sent out no less than 35 parties for the 

 observation of solar eclipses and transits of 

 the inferior planets,which work has required 

 the occupation of stations in every conti- 

 nent and Polynesia. The variations of 

 latitude have been determined at three sta- 

 tions, each one having been occupied more 

 than a year. 



MISCELLANEOUS OPERATIONS. 



The legitimate field of investigation in a 

 geodetic service embraces many subjects 

 outside of those already specified. In the 

 execution of the task before us a free inter- 

 pretation has been given to the law author- 

 izing the work, and the kindred subjects of 

 Hypsometrjr, Magnetism, Gravity and Phys- 

 ical Hydrography have been pursued along 

 with others more strictly within our prov- 

 ince. 



Five thousand miles (8,0'i7 kilometers) 

 of precise levelling have been executed, in- 

 cluding four independent determinations of 

 the height of St. Louis. Two have been 

 made from the Atlantic at Sandy Hook, 

 and two from the Gulf of Mexico at Biloxi. 



A comparison indicates that the surface 

 of the Gulf is somewhat higher than the 

 sea level at New York, and this has been 

 verified in character, although not precisely 

 in amount, by a line across the peninsula of 

 Florida, three times repeated. Other sub- 

 sidiary lines have been observed. The limit 

 of error has been that usually adopted in 

 similar work, viz., 5 mm. .y/K. The heights 

 by spirit level have been supplemented and 

 controlled by micrometric measurements 

 of zenith distances. In the determination 

 of elevations necessary to reduce the base 

 lines along the transcontinental arc to sea 

 level the latter method has been employed 

 across the Allegheny and Eocky Mountains. 

 The spirit levels are continuous from Sandy 

 Hook to Denver and Colorado Springs. 



