THE METEOR OF JULY IStII, 1846, 251 



or 18 miles ; and as the greatest angular elevation of the body at the lat- 

 ter place was about 65°, we are enabled, by a simple statement in trigo- 

 nometry, to determine its height, which is thus found to have been about 

 62 miles. A correspondent of the Philadelphia Ledger asserts that the 

 elevation of the meteor when west of Philadelphia was 42^, which would 

 indicate a distance from the earth, differing less than a mile from the 

 above determination. The data, furnished by a private correspondent at 

 Newark, Del., give also very nearly the same result. 



The apparent diameter of the body has been variously estimated at 

 from I to 6- that of the full moon. Admitting it to have been | , or eight 

 minutes, the true diameter must have been about 720 feet, or rather more 

 than a furlong. The apparent length of the tail, it is agreed, was at 

 least one degree • consequently its true length exceeded a mile. 



The first appearance of the body as seen from Lancaster was at a 

 point about 30° or 35° above tlie South-western horizon, and its disap- 

 pearance occurred at the same elevation in the North-west. The length 

 of its track was therefore rather more than 200 miles. Supposing it to 

 have been visible 16 seconds, (and this is less than the general estimate, 

 although it exceeds some others,) its velocity was about 13 miles per se- 

 cond, or two thirds that of the earth in its orbit. 



But what was the nature of the biilliant train by which the meteoric 

 body was attended ? Was it one continuous blaze proceeding from the 

 ignited nucleus } or was it merely an optical illusion, similar to that by 

 which we see an unbroken ring of light when any luminous object is 

 caused to revolve with great rapidity in a circle ? That the latter was 

 the case, will, I think, be clearly established by the following facts : 

 The impression which any visible object makes upon the fibres of the 

 retina is retained for a very short time after the object itself is removed. 

 Now in the case of the revolving luminous body, it has been found by 

 experiment, that when it makes less than ten revolutions in a second, 

 the ling is interrupted. The retina then preserves the impression about 

 one tenth of a second. Any body, therefore, moving in a straiglit line, 

 would be visible at the same instant in every part of its track through 

 which it passed in that time; and as the length of the train, according 

 to the above determination, is very nearly equal to the distance passed 

 through by the nucleus in the tenth of a second, it may be fairly in- 

 ferred that this appearance was nothing more than the optical cfToct of 

 the body's I'apid motion. 



1 have been assured by persons in Harford county, Md., Chanceford, 

 York county. Pa., and in York borough, that veiy shortly after the dis- 

 appearance of tlic meteor a distinct report like that of a distant cannon 



