C. Abbe on the Repsold Portable Circle. 211 
geodesy, whilst the great saving of time when they are used in 
the vertical of Polaris gives these instruments a remarkable su- 
periority over the meridian transit, affording a full confirmation 
of the thesis propounded on page 18 of the above named i 
by Mr. Déllen, “under all circumstances, the determination of time 
for any given instant will be best made by mounting the porta- 
ble transit, not in the plane of the meridian, but in the vertical of 
the pole star.” Mr. Brauer has intimated to me his intention to 
place his sixth transit on exhibition at the Paris Exposition. Nor 
can I here refrain from expressing my conviction of the great 
importance to the interests of our astronomy and surveying to 
be attached to the establishment in this country of a mechanical 
institute, which, under the charge of a person of the experience 
of Brauer and the Repsolds, shall be able to furnish us wit 
measuring instruments comparable with the objectives produced 
Y our opticians, and fitted to do the fine work in which the as- 
tronomer and geodesist are so much interested. —- 
For the determination of latitude the same portable transit, by 
being established in the plane of the prime vertical, yields ze- 
nith distances whose accuracy is in general much greater than 
that of the declinations of the stars observed, since these are in 
general faint ones. The portable prime vertical transit loses 
therefore somewhat of its importance, excepting for determin- 
ations of latitude where the highest accuracy is sought, and 
when by the codperation of fixed observatories, special simulta- 
neous investigations into the declinations of the stars observed 
can be obtained. 
In general, the use of the prime vertical transit restricts one 
to the observation of a limited number of faint stars situated in 
a narrow belt. The use of the Talcott zenith telescope demands 
accuracy. The portable vertical circle offers itself to us as aD | 
instrument equally applicable in all latitudes to the determina- 
tion of latitude, and fit for the determination of time for secon- 
tions between 70° of latitude and the equator, if accu- 
Tately constructed as by Repsold, and used so as to eliminate 
