estion there may be of durability
(upon which the point of economy hinges), so far as the interest on
the increased cost due to rubber tires, is offset against the greater
wear and tear of iron rimmed wheels. It is stated, on good authority
that a rubber tired engine, started at work in Aberdeen, Scotland,
wore out its tires between April and September, inclusive, and when
it is taken into consideration, that the cost of these tires is
about half that of other engines, made with solid iron rimmed driving
wheels, it will be seen that, unless very much greater durability than
this can be shown for the rubber, the advantages of such tires are
very nearly, if not more than, balanced by their disadvantages.

The fact that one set of tires wore out so soon does not prove a rule.
There may have been causes at work which do not affect such tires
generally, and it would be, we think, quite premature to form
favorable or unfavorable judgment, of relative economy from such data
as have been yet furnished.

The difference in the current expenses of running the two most
prominent types of engines, with hard and soft tires, now in use, does
not affect the question of rubber tires, unless it can be shown that
these tires necessitate, _per se_, such a form of engine as requires
a greater consumption of fuel, and greater cost of attendance, to
perform a given amount of work.

* * * * *

CENTRAL SHAFT OF THE HOOSAC TUNNEL.

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,