VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 525 



venture to reject it, however improbable it may be thought, but leaves it as a 

 point to be cleared up by future observers. 



§ 7. To determine the bulk of the fire-ball, we must not only have calculated 

 its distance, but also know the angle under which it appeared. For this pur- 

 pose the moon is the usual term of comparison ; but as it was thought, at very 

 different distances, to present a disc equal to that luminary's, and the same 

 expressions have been applied to most preceding fire-balls, Dr. B. conceives this 

 estimation rather to be a general effect of the strong impression produced by 

 such splendid objects on the mind, than to convey any determinate idea of their 

 size. However, if we suppose its transverse diameter to have subtended an 

 angle of 30' when it passed over the zenith, which probably is not very wide of 

 the truth, and that it was 50 miles high, it must have been almost half a mile 

 across. The tail sometimes appeared 10 or 12 times longer than the body; but 

 most of this was train, and the real elongation behind seems seldom to have 

 exceeded twice or thrice its transverse diameter, consequently was between 1 and 

 2 miles long. Now if the cubical contents be considered, for it appeared equally 

 round and full in all directions, such an enormous mass, moving with extreme 

 velocity, affords just matter of astonishment. 



§ 8. The duration of the meteor is very differently stated, partly because 

 some observers had it in view a much longer time than others, and partly because 

 they formed different judgments of the time. Those who saw least of it seem 

 to have perceived its illumination about 10 seconds, and those who saw most of 

 it about a minute : hence the various accounts may in some measure be recon- 

 ciled. Mr. Herschel, f. r. s., at Windsor, must have kept it in sight long after 

 other observers had thought it extinct : for though probably he did not see the 

 beginning, as it never appeared to him like a single ball, he watched it as much 

 as " 40 or 45 seconds, the last 20 or 25 of which it remained almost in one 

 situation, within a few degrees of the horizon." This confirms the foreign 

 accounts of its long progress to the southward. 



^ y. From the apparent motion of the meteor compared with its height, some 

 computation may be formed of its astonishing velocity. As at the height of 50 

 miles above the surface of the earth, it might be visible from the same station for 

 a tract of more than 1200 miles, and the longest continuance of its illumination 

 scarcely exceeded a minute, we have hence some presumption that it moved not 

 less than 20 miles in a second. The Rev. Mr. Watson, in his letter to Lord 

 Mulgrave, says, that the arc described by it while in his view could not be less 

 than 70° or 80°, and yet the time could not exceed 4 s , or 5 s at most : this, with 

 an altitude of 60°, and height of 50 miles, gives for its velocity about 21 miles 

 in a second. The observer at Newton Ardes estimated its motion to be 30 miles 

 in a second. Mr. Herschel found it describe an arch of 167 during the 40 or 



