PHYSICAL ASTRONOMY. 565 



that Double Stars are held together by a central force, to prove that 

 this force follows a different law from the only law which has hitherto 

 been found to obtain in the universe, and which obtains between all 

 the known masses of the universe, would require very clear and distinct 

 evidence, of which astronomers have as yet seen no trace. 



CHAPTER VI. 



Sect. 1. Instruments. 2. Clocks. 



IN page 473, I have described the manner in which astronomers are 

 able to observe the transit of a star, and other astronomical phe- 

 nomena, to the exactness of a tenth of a second of time. The mode 

 of observation there described implies that the observer at the moment 

 of observation compares the impressions of the eye and of the ear. 

 Now it is found that the habit which the observer must form of doing 

 this operates differently in different observers, so that one observer 

 notes the same fact as happening a fraction of a second earlier or later 

 than another observer does ; and this in every case. Thus, using the 

 term equation, as we use it in Astronomy, to express a correction by 

 which we get regularity from irregularity, there is a personal equation 

 belonging to this mode of observation, ehowing that it is liable to error. 

 Can this error be got rid of? 



It is at any rate much diminished by a method of observation re- 

 cently introduced into observatories, and first practised in America. 

 The essential feature of this mode of observation consists in combining 

 the impression of sight with that of touch, instead of with that of 

 hearing. The observer at the moment of observation presses with his 

 finger so as to make a mark on a machine which by its motion meas- 

 ures time with great accuracy and on a large scale ; and thus small 

 intervals of time are made visible. 



A universal, though not a necessary, part of .this machinery, as 

 hitherto adopted, is, that a galvanic circuit has been employed in 

 conveying the impression from the finger to the part where time is 

 measured and marked. The facility with which galvanic wires cau 



