The daily exchange. [volume] (Baltimore, Md.) 1858-1861, 1858-04-23
SCIENCE AND ART

PHOTOGRAPHY OF THE MOoN.-Besides the pictures
taken in own country-which, however, are al
most valueless as moon maps, the sides are re-
versed in the copying from the daguerreotvpe plate
upon which they were originally taken-the moon
has been photographed by several distinguished
European astronomers, including Professor Phil
lips, NJ. Secch, MN. Bertsch and Arnaud, Mr..
Hartnup and Others Prof Phillips telescope has
a siderea focus of eleven feet. and an aperture of
six and one-fourth inches: consequently, the bril-
liancy of the moons image in its focus is augmented
twenty-siX times what she appears to the
naked eye. The average time occupied for the
colodlon plate to receive the impression was about
three minutes. NJ Secchis telescope having
sidereal focus of eighteen times its aperture, the
moons image is intensified 37S times, and the time
required for the impression is an average of six
minutes. NJ. Poro's glass of forty mine feet side
real focus and twenty inches aperture, gives a
moon image 1913 times brighter than she appears to
the naked eve, and the average time of taking the
picture is seventeen seconds. Mr.. Hartnup's tele-
scope being twelve and One-half feet focus and
eight inches aperture augments the intensity of the
moons image at its focus 35. times The rapidity
with which these pictures taken may be bet
ter understood by comparing them with those of
terrestrial objects under similar circumstances. 1t
is known that the actual illumination of the lunar
surface is not much superior to that of weathered
sandstone rock in full sunshine. And, by compar-
ing the obect-glass to power with the ordinary
camera lens its focal length being nearly nineteen
times the aperture, and the moons image being
copied by its means in four seconds, it is found to
be equivalent to copying sandstone Illuminated by
the lin four seconds with lens four and a half
inches focus, and little less than quarter inch
diaphraghm.
New RAILWAY SGGNAL.-A new railway signal has
been satisfactorily tested in England, and is coming
into general use. The qualities which it possesses,
and which are relied on establishing its value and
efficiency, are-that the signal can be made from
the guard to the driver, and back again, with Ger
tainty and rapdity; that the guard and driver are
able to communicate with each other by means of a
code of signals In certain cases, the signal appara
tus is selfacting-au tomatc-for instance, if any
accident the ofhe train, as to pre-
vent any communication between the guard and the
driver, by the voluntary action of either then notice
of the fact will be conveyed to them by the appa-
rates ITSELF there is special skill required in or
der to manage or make the signal-that is, it is not
liable to arrangement, and, in case some derange-
ment does occur the ordinary workmen employed
on railway works are able to set the apparatus right
or make a new one; there is always constant indi
cation before the parties in charge of the train that
the signal is in working order, that the guard
will not start from the station without knowing that
the signal is all right and in reliable condition, and
that there will be serious delay in making up the
train of carriages cars because of the of the
Sirnal