At the regular weekly meeting of the Polytechnic Association of the
American Institute, held on Thursday evening, the 25th ult., the subject
of boiler clothing was discussed at some length, but without any
decisive conclusion being arrived at respecting the most serviceable and
economical material for that purpose. It appeared from the testimony
adduced, that though there is a variety of substances in use, even those
which are practically acknowledged as being the most efficient are
far from coming up to the required standard of utility, and are
characterized by defects which are at once forced upon us by a little
close examination. Felt is an admirable non-conductor of heat, but owing
to its combustible nature it is quite unreliable when subject to the
heat of a high pressure of steam. A large fragment of this material
which had been taken off the boiler of a North River steamboat was
exhibited at the meeting, scorched and charred as if it had been exposed
to the direct action of fire. For these reasons felt covering is,
generally speaking, confined to boilers in which a comparatively low
pressure of steam is maintained. But even under the most favorable
circumstances of actual wear its durability is limited to a short
period.

Powdered charcoal possesses the elements of efficiency as a
non-conductor in an eminent degree; but its susceptibility of taking
fire militates strongly against its adoption as a boiler covering.

Besides the materials above mentioned, there are some which come under
the denomination of cements; but the use of such is somewhat at variance
with what a dull world would call "facts." Employing them as a clothing
for a vessel in which it is necessary to retain heat is certainly the
wrong way of doing a light thing, if the evidence of distinguished