TRANSIT INSTRUMENT. 



as exactly in the meridian as possible. It will be seen that when 

 the axis of the transit telescope, E F, is placed due east and west, 

 and also made perfectly horizontal by means of the spirit level H, 

 the telescope A. B, will move in the meridian, i. e. t it will, if 



B 



directed tq the heavens, mark the exact situation of the meridian, 

 at the time, of the particular plact where the instrument is 

 located. We are thus furnished with the means of determining, 

 with the greatest exactness, the precise time of a siderial revolu- 

 tion of the earth, and as the apparent time of noon, or twelve 

 o'clock, is precisely the instant when the sun's centre is on the 

 meridian, we are also enabled to determine, with considerable 

 precision, the local time, or clock time at the place. 



The transit instrument and the astronomical clock, are the two 

 chief instruments of the observatory, and by their means, the 

 positions of celestial objects can be ascertained with the utmost 

 nicety. It would be out of place for us to describe more minutely 

 these invaluable aids to the astronomer, and we pass to consider 

 in the next chapter, the "Calendar," or the division of the year 

 into months, weeks, and days, and at the same time we shall give 

 an historical sketch of "its gradual progress to the present state of 

 perfe'ction. 



It is a difficult thing to comprehend fully, or even partially, the 

 relative dimensions, situation, and movement of our globe. We 

 are so accustomed to look around us and behold the solid founda- 

 tions of the earth, to see plains and oceans, extending as far as the 

 eye can reach, and man is so small, when compared with the 



