212 Miscellanies. 



Prof. Bache's account of his observations is contained in the following 

 extract of a letter which accompanied them, dated November 29, 1839. 



"The place of observation is a room in one of the out buildings for the 

 dwellings of the professors of the Girard College. As the materials used 

 in the construction of the house must produce considerable local attrac- 

 tion, no absolute measures have been attempted. All movable magnetic 

 substances were removed from the vicinity of the needle. A window 

 near the needle was carefully closed by a shutter of wood, and by two 

 curtains fastened to the window frame, and with an interval between 

 them. There is no fire in the room ; and a double door is between the 

 observing room and an adjoining one where there is a fire. 



" fte instrument is one of Gauss's declination magnetometers, made 

 by Mayerstein of Gottingen. The arrangement of it agrees exactly with 

 that described in the Resultate, which has been followed as nearly as pos- 

 sible. The reading telescope is supported upon a small wooden shelf 

 fastened to one of the side walls of the room ; the scale is attached to a 

 wooden frame before the shelf. One of the smallest divisions of the 

 scale, which is divided by estimation in the observations to tenths, is 25" 

 .975 nearly in value. The zero of the scale did not vary sensibly in posi- 

 tion during the observations. 



" The observations were made every eight seconds, an assistant striking 

 two seconds before the time of each observation. The ticks of the half- 

 seconds chronometer being distinctly audible, the observation was made 

 at the fourth beat after the signal given by the assistant, and thus the time 

 was independent of the minute accuracy of the signal. Checks were 

 adopted, to prevent or detect large errors in giving the signal. The in- 

 terval of eight seconds is very nearly one third of the time of oscillation 

 of the magnet bar. 



" The time was observed by a chronometer beating half seconds. This 

 was compared, before and after each set of magnetic observations, with 

 one, and after the morning of the 14th with two chronometers. One of 

 these was carried from the Girard College to the city after each set of ob- 

 servations ; but the others remained during the night at the Girard College, 

 and were removed to the city after the morning series, to compare them 

 with the stationary chronometer belonging to the High School Observa- 

 tory, the rate of which was ascertained by observations of transit of the 

 sun and stars on the 6th, 9th, 1 1th, 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 magnetic collimator with a graduated scale of glass, 



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



