An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

AST'rIONO(M MCAL IS'NTIWSNMIN'S., 53
froml the solar time by any quantity whllatccr. The sidereal
clock may point to 3A. 20l., in the morninfg, at noon, or any
otlel time of day, because it merely shows that 3h. 20mn. lxave
elapsed since the equinox was on the meridians. telce, wheln
a star is on the' meridianl the sidereal clock sh:ows its rig'ht ascension.
A.n astronomleial clock lmust have a compensation pendultum,
antd ble mtade as perfect as possible. fIts uniformity of movemerit can 1 be tested by the trantlsit instrument, tan ld a list of riglht
aseensions of stalrs. It is not so ilmportant that it should point
to Oh. OGi. Os. when the equinox is on the meridian, or that it
should not qazit. or lose comlpared with the revolution of thoe
starxs, as that it tshould move.anb,ly through the day, and
firol day to dayn. It is not customllary, the:refore to alter thle
clock, atfter it is once set, but to note from day to day how much
it is out of the way, and how fast it gains or loses. The first is
called tthe (',lor, the last tlhe rate. If these are known, then
the exact timle of an observation can be, obtained.'125. To observe the transit of a star, tthe eye must discern
tlhe iinstatnt of its bisection )by the wire, and the ea-rk attend to
the beat of the clock, the secondls bcilng counted froml the last
co-impleted minu'te before the observation beg'an. If the bisectionl occurs between two beats, as it comlnonly does, thle observer needts nucll pr)actice to be )able to divide the sectondl
accuratcly illto tentlls, and ecide at whllich of thein the transl' it
takes place. What is htow 1known as the..merean, m-et/f.w lt.l of
obsterving transits, and recording thtem by clcetro-lnma'geteism,
gives great facility and accttracy to this most dlieficlt alld ilmportant part of the w\ork..titfe pndultuml of tlhe olbservratory clock is arranged to close
the circuit of a battery, a. nd break it again at the beginning of
every beat. The closing of the circuit gives a small lateral
motion to the registering lpen), utlder whlich the paper is advalncinlg about ant inch per second.'thlIts the seconds are all
permanently recorded by notches one inch asunder ini at straiglht
Fig. 156,
a b A a 4..................................................................'......... s.............
line, as fa, b, e, &e. (Fig.'15'). The imark at the beginnling of
each,mi-nutdle has some peculia. rity by which it ma)y be dist il