As many of our readers have evinced much interest and ingenuity on
the question of the propriety of placing reliance upon the accuracy of
dropping a perpendicular from the top to the bottom of a shaft 1,030
feet in depth, by means of an ordinary plummet, we take the earliest
opportunity of settling the matter beyond dispute, by reporting
the results lately obtained, through a series of experiments by the
engineers in charge, for the ultimate purpose of laying down the
correct line for the tunnel.
The perpendicular line has, of course, been dropped many times, and
the main result taken. The plummet used is made of steel, properly
balanced and polished, in shape something like a pineapple, and of
about the same size, weighing fifteen pounds. It was suspended, with
the large end downwards, by a thin copper wire, one fortieth of an
inch in diameter, immersed in water; and, after careful steadying with
the hand, occupied about an hour in assuming its final position or
motion, which, contrary to the expectation and theories of many,
resulted in a circular motion around a fixed point, the diameter of
the circle being a mean of one quarter of an inch. The suspending
wire in these operations was not quite the entire length of the shaft,
being only 900 feet; and before the plummet had settled, the wire had
stretched nearly twenty feet.
The suspension of the plummet in water was not considered necessary
for any other reason than that water was continually trickling down
the wire, and dropping on the plummet. The experiments so far have
not been of the perfect character it is determined to attain, when the
final alignment is made, as, until the headings east and west of the
shaft have advanced to a considerable distance, any slight error would
be of no account.
A neat and ingenious instrument has been constructed for determining
the variation of the plummet, and will be used when great accuracy is
desired; the plummet will also be suspended in oil.
The bearing of the tunnel is about S. 81 deg. E.; but, independently of
its near approach to the line of revolution described by the earth,
it is not considered necessary to take into account any motion it may
derive from this cause. In fact, the opinion is, that the motion of
the earth will not practically have any effect.
On the whole, after the still imperfect experiments which have been
made, enough is established to show there is no difficulty to be
encountered, other than the accurate and delicate manipulation of the
plummet and its attachments.
The shaft headings are progressing favorably. The rock is not so hard
or varied as that met with at the west end markings. Already nearly
300 feet have been taken out, and with the proved energy of the
contractors, this great task will doubtless be prosecuted steadily and
surely to completion, within the contract time expiring March 1, 1874.
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