present
in the pipes has been carried to the valve and lodged there by the water,
causing it to stick, and with expansion traps not to close properly or to
work abnormally some way or other. The putting of these contrivances to
rights involves a certain amount of trouble, which is completely obviated
by the arrangement shown in the annexed engravings, which is certainly a
simple, strong, and substantial article. The foot of the trap is made of
cast iron, the seat of the valve being of gun metal, let into the
diaphragm, cast inside the hollow cylinder. The valve, D, is also of gun
metal, and passing to outside through a stuffing box is connected to the
central expansion pipe by a nut at E. The valve is set by two brass nuts
at the top, so as to be just tight when steam hot; if, then, from the
presence of water the trap is cooled, the pipe contracts and the water
escapes. A mud door is provided, by which the mud can be removed as
required. The silt or dirt that may be in the pipes is carried to the trap
by the water, and is deposited in the cavity, as shown, the water rises,
and when the valve, D, opens escapes at the pipe, F, and may be allowed to
run to waste. A pipe is not shown attached to F, but needless to say one
may be connected and led anywhere, provided the steam pressure is
sufficient. For this purpose the stuffing-box is provided; it is really
not required if the water runs to waste, as is represented in the
engraving. To give our readers some idea of the dimensions of the valve,
we may say that the smallest size of trap has 1 in. expansion pipe and a
valve 3 in. diameter, the next size 11/4 in. expansion pipe and a valve 41/2
in. diameter, and the largest size has a pipe 11/2 in. and a valve 6 in.
diameter. Altogether, the contrivance has some important practical
advantages to recommend it.--_Mech. World._

* * * * *

CRITICAL METHODS OF DETECTING ERRORS IN PLANE SURFACES.[3]

[Footnote 3: A paper read before the Engineers' Society of Western
Pennsylvania, Dec. 10, 1884.]

By JOHN A. BRASHEAR.

In our study of the exact methods of measurement in use to-day, in the
various branches of scientific investigation, we should not forget that it
has been a plant of very slow growth, and it is interesting indeed to
glance along the pathway of the past to see how step by step our micron of
to-day has been evolved from the cubit, the hand's breadth, the span, and,
if you plea