ON ASTRONOMY. 99 



or secondary "base lines are measured by means of wooden or iron bars 

 of simple construction ; but all such measurements are, in time, super- 

 seded by the progress of the primary triangulation. 



In measuring the angles of the primary triangulation theodolites 

 of various sizes and construction are employed. The largest instru- 

 ment of this class belonging to the Coast Survey has a circle of thirty 

 inches diameter. It was designed by Mr. Hassler, the first superin- 

 tendent of the Survey, and constructed by Trough ton and Simms of 

 London. The circle is graduated to five minutes, and may be read 

 to single seconds by means of micrometers. It is provided with a tele- 

 scope of great power, by which signals many miles distant may be 

 seen distinctly. This instrument is employed principally on the north- 

 east part of the coast, where the lines range from twenty to eighty 

 miles in length. The accuracy of this instrument is such that the 

 probable error of a single measure of an angle is about one second and 

 a quarter ; a mean of about thirty measures is taken for each angle. 



Another theodolite of nearly similar construction and of twenty- 

 four inches diameter is also employed in the survey. Besides these, 

 smaller repeating theodolites of various sizes are used on different 

 parts of the work. 



Various instruments have been employed for the determination of 

 the latitude of the most prominent points of the triangulation, among 

 which may be mentioned vertical circles, repeating theodolites, and 

 especially the zenith sector, designed by Mr. Airy, the astronomer- 

 royal of England; but all, except the last named, have been found 

 liable to considerable errors. The zenith sector is accurate and gives 

 good results, but its great weight and the labor attendant on its use 

 make it less available for such surveys than an instrument called 

 the zenith telescope, designed by Captain A, Talcott^ late of the United 

 States engineers. This instrument is designed to measure by means 

 of a micrometer the difference of zenith distances of stars which cul- 

 minate within a short time of each other and near the zenith, on oppo- 

 site sides of it. It is portable, accurate, and simple, and is perhaps 

 destined to supersede all others for geodetic purposes. 



In the progress of the latitude observations of the survey discrep- 

 ancies have occurred which can only be accounted for by local irregu- 

 larities in the figure and density of the earth. These irregularities 

 are still the subject of investigation. For the determination of the 

 longitudes of the various points of the survey every means which sci- 

 ence can affoid has been resorted to. All the observations obtainable 

 by eclipses, occultations, and moon culminations, made in this coun- 

 try previously to 1844, together with the simultaneous observations 

 abroad, so far as they could be found, have been collected and redu- 

 ced, and similar observations have since been continued at Cambridge, 

 Philadelphia, Washington, Cliarle^ton, and Cincinnati. All these 

 stations being connected by the electric-telegraph, their differences 

 of longitude have been determined with great accuracy, and all the 

 results are referred to Cambridge, as a common station of reference. 



The difference of longitude between Cambridge and Liverpool has 

 also been determined by means of large numbers of chronometers 



