463 
tion, no absolute measures have been attempted. All 
moveable magnetic substances were removed from the vici- 
nity 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. 
«¢ The instrument is one of Gauss’s declination magne- 
tometers, made by Meyerstein of Gottingen. The arrange- 
ment of it agrees exactly with that described in the Resultate, 
which has been followed as nearly as possible. 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 position during the 
observations. 
«‘ The observations were made every eight seconds, an 
assistant striking two seconds before the time of each obser- 
vation. 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 interval 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 Collegeto the city after each set of observations ; 
but the others remained during the night at the Girard 
College, and were removed to the city after the morning 
