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. t¥0, 
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 imstrament.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 4 
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 i in posi- 
tion during the observations 
The observations were sade every eight seconds, an assistant striking 
‘po 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 aigiioorant is very easly one nee of the time of oscillation 
; se —< ; 
. “'The time was observed by a cl inoati ing half séooniel This 
was compared, before and adie each lt magnetic observations, with 
one, and after the morning of the 14th with two chronometers. One of 
servations; but the others. remained: during the night at the Girard College, 
and were removed to the city afier. the morning series, to compare 
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 starson the 6th, 9th, ith, 18th, and 16th of November. Girard 
College is about 1770 feet west, and 1050 north of the High School by 
pees ” 
The Dublin observations were made in the Magnetical Observatory 
1t employed is of the form described some time since 10" 
m a ayant Himato: thag dnated scale of glass, 
. <a : ig 5 Et es 
“ a ae ae ee ee 
gt ih o. 18. p. 330. et seq. - - 
, ioe bas i hon | 3 
