, 1866-04-28

l.AAdsAs s..saE.eE. .eE.as..aka.AAas.fsis.
discs. Yet these arc not lesa wonderful, Or
useful. and neeessitate an equal amount Of
skill In the framing. First In order are tho
BteadStng appliances. The instrument Is
built on pillar, whose foundations are deep
laid TO keep It free from surface tremor, and
enclosed by 3 building, which Is completely
isolated from 1t, 30 as to preserve the
central column unwarped by the
swaying influences Of the wind, or
the expanding influences Of the
solar ray, Which In summer will throw
building Of TINT height nearly an ioch out of
the perpendicular by heating that sldo next
the sun more than the opposite. This point
gained, the frame work has to be equipoised
In every direction, to prevent the possibility
Of leaning ever, and then the several parts
have to be made with tho maximum of
strength to the minimum Of weight. toayoid
bending. Tia whole telescope must revolve
on double axis, that It may be capable OF
TIVE motlons Mt right angles to each other:
these combined axes being ground and fitted
in the most accurate manner, 50 that there
may be no slip or "giEe" In any position.
These two motions may be readily understood
by consideration Of the diurual movement
~ - oAAA- AA@ e<A-%SIkS - AESt4fEu On one
axis, vertically, to reach the meridian al,i
tude Of the star to be observed. Il Is then
clampcd on this axis, and revosTed on the az.
Is ofnght ascension, its aSial lin. everywhere
preserving the same angle with line passing
through the poles Of the earth. When by
this duplex movement 1t has been pointed at
a star, it is ciamped to the second axis, and
connected with q delicate system Of clock.
work which carries the instrument through
one revolution in right ascension In twenty
three hours dftY-si1 minutes, that being the
length Of the s1dereal day. By this means, as
the earth moves on its axis from west by
south, to east, the telescope keeps even
pace from east 10 west, keeping the object,
il a fixed star, always in the field Of vision as
long rs required, without any trouble to the
observer, who is thus left free to watch the
star and note the phenomena presented. In
the case Of planetary body, this appliance
does not follow II strictly, especially II 1t
should be Mercury or Venus, when the Sun Is
nearly between II aud thu earth. The orbIta3
motion Of the moon being much more rapid
than either Of these, she is much sooner car
ried out Of the field. We have above
EEetched the process OF finding stur by COM
putatton Of its position in the heavens The
process is generally little ditfercut to this
in practice, q smaller telescope. called utbc
.Gnder," with axis parallel to the great one.
being used 10 tlnd the star, when the instru-
mcnt is changed as above aud the c1oeswork
set in motion.
Ihe nccnratc reading of OF the nrcs thus
passed over, involves another very critical
pschm'hafS Aaoof. on each oil these axes Is
Plaeeo large carc!e ssiStccn md twcnty-tonr
incnes olametef,l tac circumference OF which
Is Mmale1y aisldeu' Ihose who have tried
Valnls 10 dlviae truly the circumference Of 4
circle mto nine or more exactly equal parts
with the mid Of pair Of compasscs, may
family appreciate tie herculcan labor OF
el1ectiug . subdivision SO fine that tho
intermediate spaces are scarcely perceptibIc
\ with the naked eye, and yet when brought
under the powerful miscroscope are exactly
regular, md the distance between any two,
three, seven, twelve, nineteen, &C., absolutely
equal. Two Of these microscopes are attach.
| cd TO each circle for the more minute subdi.
! vision Of the arc. These arc now almost uni,
| vcrsally substituted for tue till recently en-.
ployed vclnIer, which, although excellent In
itself, is not 10 be compared to the microscope,
except at great disadvantage. The micro.
scopes nre blocked fast to the stationary frnme
| work, aud, rs the telescope rcvolves, the di.
| visions Of the circle arc carried in review be
fore them. Some circles have no 1cs. Than
! four of these reading microscopes.
| Still another annl;ancc il brought ,nto "a
| ou1>Itior-t1,a m;cromc,e--o l,,tto 1900
mcnt of screws AM, fi1ms from t,,c wah ~ T..
sniler. Which strctch,nr across tho ao,a ~
4 v;a,v enables tho oheoE-o- ~ a. T.,,,s 49,,