478 



SCIENTIFIC RECREATIONS. 



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all the transits across the wires, and divide by five the number of them 

 we shall get at a true result. 



A good clock is also a necessary ad- 

 junct for astronomical observations, and the 

 astronomical clocks and chronometers now t 

 in use record the time with almost perfect L 

 accuracy. The improvement in telescopes, L 

 the use of micrometers, etc., have greatly 

 facilitated observations. In the transit clock 

 we have a most useful timekeeper, for the 

 ordinary clocks are not sufficiently accurate 



for Very Close Observations. The Sidereal ^g- 5i5.-Th e eyepiece of transit instrument. 



time differs from solar time, and the twenty-four hours' period is calculated 



from the moment a star passes the meridian 

 until it passes it again. The sidereal day is 

 nearly four minutes shorter than the solar day, 

 and the sidereal clock marks twenty-four hours 

 instead of twelve, like the old dial at Hampton 

 Court Palace over the inner gate. The Chrono- 

 graph has also been useful to astronomers, for 

 by " pricking off" the seconds on a roller by 

 itself, the observer can mark on the same cylin- 

 der the actual moment of transit across each 

 wire of the instrument, and on inspection the 

 moment of transit may be noted. 



Fig. 5 i6.-Astronomical quadrant. 



The Equitorial is another useful instrument, and by its means the whole 

 progress' of a star can be traced. The Equitorial consists of a telescope fixed 

 so that when it has been pointed at a certain star a clock-work movement 

 can be set in motion, which exactly corresponds with the motion of the star 

 across the heavens, and so while the star moves from its rising to setting 

 it is under observation. Thus continuous observations maybe made of that 

 particular star or comet without any jerking or irregular movement. 



We can thus see the uniform motion of the stars which go on in greater 

 or lesser circles as they are nearer to or farther from the pole ; and with 

 the exception of the polar star, which, so far as we are concerned, may be 

 considered stationary, every star moves round from east to west that is, from 

 the east of the polar star to the west of it, in an oblique direction. There- 

 fore, as Professor Airy remarks, " Either the heavens are solid, and go all of 

 a piece, or the heavens may be assumed to be fixed or immovable, and that 

 we and the earth are turning instead of them." 



The Mural Circle is another very useful instrument, and is used by 

 calling to aid the powers of reflection of quicksilver, in which a bright star 

 will appear below the horizon at the same angle as the real star above the 

 horizon, and thus the angular distance from the pole or the horizon of any 

 star can be calculated when we know the inclination of the telescope. The 



