New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1880-08-16

| degrees.
At the angle formed by the line of the two tee-
scopes and placed so as to be in the line of vision OF
both. stands small mirror Of peculiar construction.
IL is square column of steel. five inches In length
by two IN diameter and plated upon ITS four sides
with nickel. 1t is mounted top and bottom. upon
plyots, so as to revolve horizontally and 1t is to this
mirror that the raw-hide cog-wheel conveys its
power. Affixed to one of the wheels which gives
motion to the mirror is device for breaking an
electric current. connected with o recording clock,
so that the number of revolutions Of the mirror each
second is automatically recorded. The power to
give the mirror its revolutions is furnished by a
steam engine built for the purpose and placed in
adjoining building.
few feet distant from the eye-glass of the fixed
telescope, and in direct line with the instrument
is placed hehoscope-an apparatus consisting of n
mirror mounted upon complicated bearings and
moved by clock work, SO that when wound up and
adjusted il will follow the apparent motion of the
sun. rejecting its Tays for any desired length of
time upon given point. The rays. or. TO speak
scientifically, the waves, of sunlight thrown from
this instrument are brought to focus by an inter-
mediate lens upon the eye-gass of the fixed tele-
scope through which they pass to tie surface of the
revolving mirror at the other end.
At the Naval Observatory in Washington, some
Potomac hts exact distance will be ascertained by
the Coast Survey before actual work is begun, is
placed fixed Mirror circular in form, three feet in
diameter aud Of such a delicate degree of concayity
that light thrown upon it from the revolving mir
rOr at the fort is returned again and brought to
focus as near may be upon Its starIing-pont. In
other words. this mirror is a section OF the inner
surface of a shell which if complete. would be three
miles or more in diameter OF course to all appear
ance, it Is perfectly fiat.
Now, to understand the working Of the apparatus.
let IT first be supposed that the revolving mirror is
at rest The of The suns rays will be as fol-
lows: Striking the mirror of the helioscope. they
will be reflected through the intermediate lens
upon the eye-gass of the fixed telescope, through
the instrument to one side of tho revolving mirror
thence to tn concave mirror across the Potomac,
by which they are returned to the revolving mir
rOr and are finally reflected through the movable
telescope to the eye of the observer placed at the
eye-glass The point at which this telescope rests
to receive the SUMS RAYS after they have made their
tortuous journey, the revolving mirror being at
rest is marked zero. Now apply the steam and set
the revolving mirror in motion. 1t sounds like the
concentrated shrek of hundred terrified children.
Give II 900 revolutions second, and its four sides
will throw 500 reflections of the suns rays across
the Potomac to the mirror at the Observatory.
Nothing but the shrieK indicates that the mirror is
IN motion it looks like small polished cylinder.
The observer no longer catches the suns reflected
rays through the movable telescope at zero. but
swings his instrument to the right or left. ac-
cordng to the direction in which the mirror is
made to revolve, until they again reach his eye.
Reflections following each other with such enor-
nous rapidty. appear single reflection.
This then is the whole process 1t remains only to
note the number of revolutions per second, as in
dicated by the recording clock. and the degree of
DETECTION from its normal position at zero given
to the movable telescope. and work ont the result
as q sum" In arithmetic 1t will be seen that
while ray of sunlight is making its journey across
the Potomac and back, between the revolving mir
ror and the fixed concave mirror. the former Will
have made some portion of one of its revolutions
This portion is represented by the degree of defec
tion which il Is necessary to give to the movable
telescope 10 catch the rays upon their return AID
second reflection from the revolving mirror.
The following propositions, pencilled by Professor
be attained from given hypothesis: Speed Of mir
ror 900 turns per second; deviation of rejecting
raw 21so: distance, 1.6 mile. Results Motion of
mirror In l second, $50 X200=72,O ,O00o; observed
motion Of mirror wiiie light is going and coming
H. is 72,0OO-1 114=57.600 Then velocity OF
hght=2X1 1.6X57 ,600=1S4,3 800 miles per second.