184 



Review of Professor Olmsted on Meteors. 



angle from different points on the eartli's surface. Thus, if a northern 

 observer marked it by a certain star, a second observer in the south would 

 refer it to a point further north, and one in the west, further east. The 

 difference between the angle under which it was seen by the northern and 

 southern observers, would constitute parallax in declination ; between the 

 western and eastern, parallax in right ascension; and hence, the distance 

 between the points of observation being known, the height of the meteor 

 could readily be deduced. In the present instance, however, the data were 

 imperfect and sometimes discordant; so that although 2,238 miles is the 

 computed altitude, both Mr. Twining and Professor Olmsted are disposed 

 to consider the real source of the meteors as considerably more distant, 

 and at any rate, the above limit is confidently believed not to exceed the 

 actual distance. 



The meteors are supposed to have been attracted by gravity towards 

 the centre of the earth, towards which they would therefore be drawn in 

 parallel lines : such being the case, their apparent radiation from a common 

 centre may thus be accounted for. Let A, B, C, represent the vault of 

 , ., B 2 , the sky, the centre, D, 



being the place of the spec- 

 tator. Let 1, 2, 3, 4, repre- 

 sent parallel lines directed 

 towards the earth. A lumin- 

 ous body descending through 

 the line D, E, coincident with 

 c the axis of vision, would ap- 

 pear stationary all the while 

 at I'; a body descending the line 2, 2, would appear to describe the short 

 arc 2', 2'; and a body descetiding the line 4, 4, would appear to describe 

 the longer arc 4', 4', so that the arcs described would appear shorter or 

 longer according as they were nearer to or further off from a coincidence 

 with the axis of vision ; those meteors whose course nearly coincided with 

 the axis of vision, appearing to describe shorter arcs, and to move more 

 rapidly, than others at a greater distance from it, that which might happen 

 to fall exactly in the axis, being seen only as a luminous speck. 



Professor Olmsted next proceeds to enquire, •' what velocity the meteors 

 would acquire in falling from a point, 2,238 miles (their height at the 

 lowest computation) above the earth, to icithin about fifty miles from its 

 surface," this being taken aS the height of the atmosphere. This would 

 give a velocity of four miles per second, as that at which the meteors would 

 enter the atmosphere — a velocity nineteen times that of sound. Now it is 

 well known that when air is suddenly compressed, a quantity of heat is 

 extricated from it, as we see in the common instrument for lighting tinder 

 upon this principle : so that the meteors, from the rapidity with which 

 they were moving upon entering the atmosphere, would produce a sudden 



I 



