1835.] On the velocity of light. 291 



practice the contrary has in several instances come un- 

 der the notice of the present writer j a superficial know- 

 ledge has given rise to curiosity and enquiry, and en- 

 quiry has led to study, and thus has led to a knowledge, 

 which but for the curiosity and enquiry excited would 

 never have been attained ; with this position by way of 

 apology, I propose in the following pages (without any 

 attempt at the analytical investigation) to offer some re- 

 marks upon the method employed by Astronomers for 

 discovering the velocity of hght. There are few or none 

 of your readers perhaps who have not noticed that a sen- 

 sible interval of time elapses between the flash of a gun as 

 seen from a distance and the report; or betv/een a flash 

 of lightning and the consequent peal of thunder v/hich 

 follows; but few there are I apprehend who are ei'iiio 

 that an interval (a comparatively small one it i.;; true) 

 occurs between the time when the gun actually did flash 

 and when the flash teas seen by an observer situated at 

 a distance : it is true that for any distance on the sur- 

 face of our Planet the interval is quite inappreciable since 

 it occupies only the twenty-fifth part of a second of time 

 to pass over a distance equal to the Earth's diameter ; 

 hence were our observations confined to terrestrial ob- 

 jects only, the question of the velocity of light Avould 

 probably never have occupied the attention of the learn- 

 ed, since it never would have been conjectured or even 

 admitted without a proof, that on looking at the sun or 

 any other heavenly body we do not see it in the situa- 

 tion it actually occupies at the moment we see it— sup- 

 pose for example that a rocket was fired on the surface 

 of the Moon, or that a volcano were to burst out— the 

 phenomena would not be seen by a spectator on the sur- 

 face of the earth till one second and six tenths after the 

 actual time of bursting, and were the same to occur up- 

 on the surface of the sun, the time at which it would bo 

 observed on the earth would not be till S minutes and 1:^ 



