in Biding his Test-Plates. By William A. Bogers. 85 
In order to give a motion in revolution to the holder, for the pur- 
pose of grinding circular points, a Hook joint is used to connect it 
with a driving pulley. 
It may be proper at this point to offer a few observations, 
derived from experience, on the kind and quality of glass best suited 
to receive delicate lines. I have previously made some remarks 
before the Academy on what, for the want of a better term, was 
described as the grain of polished crown glass. Subsequent ob- 
servations have not entirely confirmed the views expressed at that 
time. Still, there does not seem to be much doubt but that 
certain kinds of glass are capable of receiving perfect lines only in 
one direction. When the lines are ruled at an angle with the 
general direction of the grain, the edges at once become serrated 
if they are very fine; whereas, if they are coarse, they either 
become enlarged throughout their whole length, the edges remain- 
ing smooth, or else they wholly break up, presenting a very ragged 
appearance. If the lines are as fine as 25,000 or 30,000 to the 
inch, this delicately serrated appearance can be detected at once ; 
whereas, if the lines are coarse, several days may elapse before 
the tension by which the particles seem to be held together, is 
broken. 
Two instances occurring in my own experience may serve to 
illustrate this action. In one, while I was examining a set of lines 
some days after they were ruled, I was fortunate in seeing two or 
three lines enlarge throughout their whole length. From being fine 
lines, they became, almost in an instant, very heavy lines, smooth, 
black, and of excellent quality every way. The action of breaking 
up was just slow enough to enable me to follow it. In the other, 
the lines had been ruled about two weeks ; and for protection they 
were covered with microscopic glass, closely cemented to the surface. 
During my examination, the whole surface became completely 
broken up. Such was the force of the explosion, that particles of 
glass yoVo of i^ch in length were driven a distance of y^o of an 
inch. In fact, the debris covered the whole surface of the glass 
under examination. All the particles presented a curved appear- 
ance ; and, with hardly an exception, the curvature was always 
in the same direction. On both of these plates lines were 
afterwards ruled in an opposite direction, but without noticeable 
results. 
It may be said that this phenomenon was due to the pecuhar 
action of the cutting crystal with respect to the surface of the 
glass ; but in all subsequent experiments, in which similar but 
less striking results were noticed, lines were ruled in both directions 
at the same time, and under the same conditions. In the case of a 
particular importation of polished crown glass from Chance and Sons, 
the evidence of grain is so marked and of such constant recurrence, 
