120 PllOCEEDINGS OF THE AMERICAN ACADEMY 



pies from the Coast Survey observations. From this pubUcation, it 

 appears that the latitude is given, by a single night of observation, to 

 the fraction of a second of arc, and that in four or five nights it 

 can be determined with the minutest accuracy of vi'hich astronomical 

 measurement is susceptible. The instrument employed is of simple 

 construction, and of little cost, while its accuracy must render it avail- 

 able for some most delicate geodetic and geological researches. It is 

 not impossible that, in the hands of a skilful geologist, it may aid in 

 determining the various densities of the crust of our globe, and thereby 

 serve as a divining-rod for detecting its internal wealth ; and it may 

 thus give birth to a new species of practical astrology. 



" The use of Morse's magnetic telegraph for the determination of 

 astronomical differences of longitude was too obvious to escape early 

 notice, but it was reserved for the Coast Survey to ascertain its practi- 

 cability as an exact method. This has been done by a series of refined 

 and careful observations, made under the direction of Mr. Sears C. 

 Walker, from which it appears that differences of longitude thus deter- 

 mined can be employed in the measurement of the earth in a direction 

 perpendicular to the meridian. This conclusion is of great importance 

 in reference to the survey of a coast which deviates from the arc of a 

 meridian so much as that of the United States. The introduction of 

 clockwork into the magnetic operations will undoubtedly contribute to 

 their accuracy, although it remains to be seen which of the different 

 plans that have been devised will be the preferable one. 



" Professor Bache's method of employing the great theodolite in the 

 primary triangulation must command the admiration of experienced 

 observers for its conscientious accuracy, and its skilful and faithful 

 determination of every correction and every source of error. His 

 measured angles have rewarded his patience and perseverance, by 

 submitting to the usual tests with a uniform exactness which has never 

 been surpassed, and which proves that one fifth of a second of arc is 

 the greatest error to which any one of his angles is liable. This extra- 

 ordinary accuracy is not attained at any sacrifice of time'; but, on the 

 contrary, the present Superintendent has completed the observations at 

 each station in much less time than was required by his predecessor, 

 because he has rejected an unnecessary and inju(!!icious rule in regard 

 to the selection of days of observation. 



" All the other portions of the field work, whether of the recon- 

 noissance, of the primary, secondary, or tertiary triangulation, or of the 



