102 LECIUEES 



be established in order to produce the effects that we observe, is for the 

 present an unsolved question. 



" The telegraphic determinations of longitude have been extended 

 from Washington northward to Philadelphia, New York, Cambridge, 

 Bangor, and Halifax, and southward to Petersburg, Wilmington, 

 Charleston, Savannah, Mobile, and New Orleans. 



" Great credit is due to the public spirit of the telegraph companies, 

 who have extended every facility to the operations of the Coast Survey, 

 and have given the use of the line after working hours free of charge. 



" In sections of the coast to which the telegraph has not yet pene- 

 trated, such as Florida, Texas, and the Pacific coast, the longitudes of 

 cardinal points are determined by observations of moon culminations 

 and by chronometer. 



' ' Corresponding observations of moon culminations are made at several 

 American observatories, at the expense of the Coast Survey ; and most 

 valuable aid is derived from the series of meridian observations of the 

 moon made at Greenwich, which are always most promptly obtained 

 through the kindness of the astronomer-royal. The manner in which 

 the reductions are made to keep pace with the observations at the 

 latter observatory is admirable, and wortliy of general imitation. 



"The chronometric determination of longitude between Savannah 

 and Fernandina, in Florida, the details of which have been communi- 

 cated to the American Association at the Montreal meeting, may serve, 

 in plan of execution and mode of discussion, as a model tor operations 

 of a similar character. 



^' The triangulation of the eastern States will furnish a measurement 

 of one arc of meridian of over three degrees in length between the 

 island of Nantucket and Mount Blue, in Maine ; another of equal ex- 

 tent along the Chesapeake Bay and southward ; and, by the admirable 

 method of determining differences of longitude by the electric tele- 

 graph an arc of parallel of latitude embracing over ten degrees of lon- 

 gitude, may be measured along the Gulf of Mexico." 



These measurements cannot fail to be of great importance in estab- 

 lishing correct values for the elements of the earth's dimensions, and 

 the result will be one of those valuable contributions to science which 

 attend the thorough prosecution of works of this character. 



The results of all the measurements in different parts of the earth 

 indicate a lengthening of the degrees as we go towards the pole ; 

 but there are many discrepancies, some, no doubt, the result of 

 the imperfection of instruments and the unavoidable error of observa- 

 tion. But there are still others which are not probably traceable to 

 these sources. They clearly show that the earth is flattened at the 

 poles, but they also show it has irregularities of figure which are not 

 yet fully explained. If we take the mean results of all the best mea- 

 surements, we have for the length of a degree at the equator 68.7 

 miles, nearly 362,626 feet ; at the arctic circle, 66° 20', 69.4 miles, 

 nearly 365,744 feet ; difference 3,118 feet. 



15. With the measured length of the degrees it is easy to obtain 

 the exact figure of the earth. 



Thus, fig. 5^ if A B is a measured arc of one degree near the pole, 



