ASTRONOMY. 11 



The better the machines for measuring time are con- 

 structed, the more nearly are they found to agree 

 with the motion of the heavens. By this the uni- 

 formity of both motions is ascertained. 



The siderial day is divided into 24> hours ; and as the 

 whole circumference of the equator passes in that 

 time over the meridian of any place, an arch of 15 

 passes in an hour ; of 1 , in four minutes ; of 1' in 

 4 seconds of time; and so in proportion. 



,18. To compare the period of a clock with the 

 length of the siderial day. 



Observe the hour at which a star comes to the meri- 

 dian on any two successive days ; the interval 

 ought to be 24? hours ; if it is more, the clock goes 

 too fast ; if less, too slow. By repeating these ob- 

 servations day after day, the rate of the clock, or 

 the quantity by which it daily advances on true 

 time, or falls short of it, will be found. By short- 

 ening the pendulum if the clock goes too slow, or 

 lengthening it if the clock goes too fast, the mo- 

 tion of the clock may be brought nearly to agree 

 with that of the stars. 



19- The period of the clock being thus adjust- 

 ed, it is required to find what hour the clock 

 marks, when the point of the vernal equinox is 

 on the meridian of a given place. 



Observe 



