ASTRONOMICAL INSTRUMENTS. 181 



supported on a central axis at right angles to its length, with care- 

 fully finished pivots, which rest on supports called Y's, and the 

 circle which is attached to one end or other of the axis is read 

 by microscopes armed with micrometers. Such an instrument can ' 

 be well studied by means of the model of the transit instrument 

 at Greenwich (No. 1,780). A vertical circle only is required for 

 transit instruments and prime vertical instruments, but where 

 azimuths are also required, the system, as above described, is made 

 to rotate on a vertical axis, and there is a horizontal circle similar 

 to the vertical one. Dr. Bruhns' Transit Instrument (No. 1,770) 

 shows how the introduction ^of the use of a prism in front of the 

 object glass affects this method of mounting, and indicates a new 

 method which is certain to be largely used in the future. 



The most perfectly-mounted instruments, however, would be 

 almost worthless, for the purposes of mechanical astronomy, were 

 the positions which they determine not accompanied by an accurate 

 statement of the time of the observations. There is ample 

 material in the Collection to show that this is now possible. 



The rude clocks of the Tychonic period have now been re- 

 placed by time-keepers only just short of absolute perfection ; the 

 compensation of the pendulum o the clock or the balance of the 

 chronometer, for changes ot temperature, is now accomplished in 

 various ways, and even the irregularity of a clock's rate for changes 

 of atmospheric pressure has now been corrected. This perfect flow 

 of time, moreover, is now electrically recorded in a permanent 

 manner by means of chronographs (No. 1,843), an d tne " e ye and 

 ear " method of judging of small intervals, by mentally dividing the 

 intervals between the beats of a seconds pendulum into tenths, is 

 now superseded by another, which enables us to record permanently, 

 as accurately as anything human can, any instant or interval of 

 time on a scale which may be as large almost as we please. If 

 observers were infallible, a thousandth of a second would now be 

 a gross quantity (Nos. 1,871 5). 



By the introduction of electricity not only can the beats of a 



