464 



series, to compare them with the stationary chronometer 

 belonging to the High School Observatory, the rate of which 

 was ascertained by observations of transit of the sun and 

 stars on the 6th, 9th, 11th, 13th, and 16th of November. 

 Girard College is about 1770 feet west, and 8050 north of 

 the High School by the city map." 



The Dubhn observations were made in the Magnetical 

 Observatory. The instrument employed is of the forn\ 

 described some time since to the Academy.* It is a mag- 

 netic collimator with a graduated scale of glass, each division 

 of which corresponds to 43''*22 of arc. The visual angle 

 under which each division is seen is so considerable, that the 

 divisions can readily be subdivided into tenths by estimation. 

 The time of vibration of the magnet is 17^*78. The appa- 

 ratus containing the magnet, as well as the reading telescope, 

 are supported on stone pillars resting on solid masonry, and 

 insulated from the floor. 



The observations having been undertaken by Prof. Lloyd 

 without assistance, it was found impracticable to observe at in- 

 tervals shorter than the time of vibration of the magnet bar, 

 of which each successive elongation was accordingly noted. 

 The tinae shown by the chronometer was usually noted every 

 tenth or twelfth vibration ; and thus the time of the interven- 

 ing observations could be interpolated with much exactness. 

 The error of the chronometer was obtained on the nights of 

 the 11th, 14th, and 19th of November, by transit observa- 

 tions with the four-foot transit of the Observatory. 



Of these observations, those made on Wednesday, 

 Nov. 13, (8 — 9) p. M. Greenwich mean time, were the most 

 favourable for the purpose contemplated. The changes, 

 though small, (from 5" to 50",) were marked and rapid, the 

 intervals of the successive maxima and minima averaging 

 thirty-six seconds. The epoch of their occurrence seems to 

 be determinable to between four and five seconds. 



* See Proceedings, No. 18, p. 330, et seq. 



