288 HISTORY OF ASTRONOMY. 



nometers which are carried to sea should be determined 

 with the utmost precision. In every commercial city 

 there are private establishments where this duty is reg- 

 ularly attended to. A public observatory does not 

 necessarily interfere with these private establishments, 

 but affords the means of rendering them more accurate 

 and efficient. How this may be accomplished will 

 presently be shown. 



An exact knowledge of time is also of vital importance 

 to the conductors of all railroad trains. A small error in 

 a conductor's watch has repeatedly been the occasion of 

 the collision of railroad trains, and the consequent de- 

 struction of human life. Many of the railroad companies 

 in this country incur annually considerable expense to 

 provide all the conductors with correct time. 



An accurate knowledge of time is important to all 

 business men, but especially to banking and other houses 

 where business is entirely suspended at a fixed hour of 

 the day. A small mistake in the time might occasion 

 not only serious disappointment, but also pecuniary loss. 



An astronomical observatory is furnished with clocks 

 of the best construction, and with transit instruments for 

 determining daily the error of these clocks. The observ- 

 atory, therefore, can furnish time with all the precision 

 which can be desired ; but to render this knowledge con- 

 veniently accessible to the public, requires some peculiar 

 arrangements. The following is the arrangement for this 

 purpose which existed for many years at Greenwich ob- 

 servatory : On one of the turrets of the observatory is 





