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 Dublin observations were made in the Magnetical 
Observatory. The instrument employed is of the form 
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 are. 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 time 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, 
Noy. 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. 
