The Locomotive - Lighthouse Survival Blog

1893.J THE LOCOMOTIVE. 45
slmpc, and jji-itty rcrtuiii to leave it in siuli a |)criiiaiiciitly strained condition that a
sliglit blow will break it readily. Of coiiise the ehillinj,' in such a easc! is much more
violent than it is in boilers, but the action is probably similar, diirering in degree ratlier
than in kind.
It may be interesting to examine tlic eilect of sudden and local t(Mnperaturc
changes in glass, a body that is notoriously apt to l)reak when suddenly lieated or
cooled. Tlie method of calculation that wc have used above may be summed nj) briefly
in the following forinulic :
o
S— C X f X Af: or t =-^ TV
C X M
where C is the coetticicnt of expansion, M the modulus of elasticity, t the sudden
change in temperature, and jS the strain per square inch produced by this cliano-e in
temperature. In the case of glass we may take C .00000.7 per degree Fahrenlieit,
and M — 8,000,000 pounds per square inch. We may also take the tensile strengtli of
glass as !i, 400 pounds per square inch, and by substituting this for N in the second of
the foregoing formukc we can find out what change in temperature will break the
glass. Thus
* ~ TOUOOOo X 8,000,000 "" ^^^'* *'''^'
Any kitchen girl who has had experience in washing glasses could tell us that this
estimate is not far from right. Indeed, glas.ses often break with a much smaller change
in temperature, a fact that is sufficiently explained by their shape, but we cannot enter
further into the mechanics of "this question in the present article. It appears that
imless glass has been specially annealed, as it is in making beakers and test tubes and
the better grades of lamp chimneys, the elastic yielding of the material surrounding the
chilled spot, which we took some account of in the case of steel, does not occur to any
great extent, so that the tbnnula gives a fairly accurate result without taking
it into account.
The Just Perceptible Difference.
A lecture was delivered on Friday, January 27th, by Mr. F. Galton on the " Just
Perceptible Difference," in which he pointed out that each sense organ has its own
internal activities which are too faint to be perceptil)le in health, but which are not
unfrequently perceived in illness, as, for example, ringing in the ear and ])eculiar taste
or smell. Their unfelt effects might, however, concur with an ordinary sense impression
and intensify it. He dwelt on the influence of the imagination and adduced
instances where imagination produced effects that had been mistaken for faint sensations
and sometimes for plainly perce])tible ones. One of the most suitable sul)jects for such
experiments he found to be the auditory imagination associated with the words perused
by the eye. He described experiments frequently made by himself at meetings of
scientific societies, where he had obtained unrevised copies in print of the papers about
to be read. Owing to some deafness he often found himself able to follow every word
distinctly only so long as his eyes rested on the paper ; he could detect the fact of any
alteration in the Avording, but was quite unable to make out the substituted words.
On these occasions when he raised his eyes from the ])a|)er he could not follow the
reader at all. He usually found it necessary to approach him by one quarter of the
previous distance, in order to follow his voice by means of the ear alone. Hence the
power of the imagination plan the power of the hearing bore the same relation to the