CLOCKS. 479 



Transit Circle is also used for this purpose, and is a combination of the transit 

 instrument with the circle. In all calculations allowance must be made for 

 refraction, for which a " Table of Refractions " has been compiled. From 

 the zenith to the horizon refraction increases. The effect of refraction can 

 be imagined, for when we see the sun apparently touching the horizon the 

 orb is really below it, for the refraction of the rays by the air apparently 

 raises the disc. 



The clock and chronometer are both very useful as well as very common 

 objects, but a brief description of the pendulum and the clock may fitly close 

 our remarks upon astronomical apparatus and instruments. The telescope 

 has been already described in a previous portion of this work, so no more 

 than a passing reference to it has been considered necessary. We therefore 

 pass on to a consideration of the measurement of time, so important to all 

 astronomers and to the public generally. 



Time was measured by the ancients by dividing the day and night 

 into twelve hours each, then by sun-dials and water-clocks, or clepsydra, and 

 sand-clocks. The stars were the timekeepers for night before any mechanical 

 means of measurement were invented. 



" What is the star now passing ? 

 The Pleiades show themselves in the east, 

 The eagle is soaring in the summit of heaven." EURIPIDES. 



Sun-dials were in use in Elijah's time, and the reference to the miracle 

 of the sun's shadow going back on the dial as a guarantee to Hezekiah, will 

 be recalled at once by our readers. These dials were universal, and till 

 sunset answered the purpose. But the hours must have been very varying, 

 and on cloudy days the sun-dials were practically useless. 



The water-clocks measured time by the dripping or flow of water, and 

 they were used to determine the duration of speeches, for orators were each 

 allotted a certain time if a number- of debaters were present. This method 

 might perhaps be adapted to the House of Commons, and speaking by the 

 clock might supersede clbture. We find allusions to these practices in the 

 orations of Demosthenes. Even this system was open to objection, for the 

 vases were frequently tampered with, and an illiberal or objectionable person 

 was mulcted of a portion of the water, while a generous or popular adversary 

 had his clepsydra brimming full. Some of these water-clocks were of elegant 

 design, and a Cupid marked the time with arrows on the column of the clock 

 of Ctesibius, while another weeping kept up the supply of water. The motive 

 power was the water, which filled a wheel-trough in a certain time, and when 

 full this trough turned over, and another was filled. The wheel revolved once 

 in six days; and by a series of pinions and wheels the movements were com- 

 municated more slowly to the pillar on which the time was marked for 360 

 days, or with other arrangements for twenty-four hours. 



The repeating of psalms by monks also marked the time, for by 

 practice a monk could tell pretty accurately how many paternosters or other 



