the Luminiferous Oilier. 413 



95 miles. But the conditions in nature are not statical. The 

 changes in temperature in the column will be continually in- 

 creasing or decreasing its height ; the air-currents also operate 

 to change it, first by increasing or decreasing the temperature 

 from the mean at considerable heights, and, secondly, by 

 operating dynamically to push the top of the column upward; 

 the aerial tides may operate to raise the column still higher, 

 and the molecules themselves are supposed to be flying with 

 great rapidity in all directions. An increase of temperature 

 of one tenth the mean value, which at the earth's surface 

 ' would be about 49° F., would elongate the column about ten 

 miles, and a corresponding decrease would shorten it about 

 the same amount, making it 105 miles in the former case and 

 85 miles in the latter. The effect of air-currents and aerial 

 tides cannot be so definitely calculated; but it is safe to assume 

 that they may produce a much greater increase of height 

 above the mean than they will depression below the mean ; 

 just as in a highly agitated sea, the depressions below the 

 mean surface-level may be small compared with the height 

 above the same level to which the spray from the top of a 

 wave may be thrown. It seems possible, therefore, that when 

 the temperature, air-currents, and aerial tides conspire to de- 

 press the column, the extreme height of the atmosphere may 

 be reduced to less than 85 miles ; and when they conspire to 

 elevate it, it may possibly rise to a height exceeding 120 

 miles. 



If it be certain, as is assumed, that the meteors are rendered 

 incandescent by atmospheric friction, and the extreme height 

 at which they are visible could be determined by direct obser- 

 vation, it would fix a height less than the extreme height of 

 the atmosphere, independent of other physical considerations; 

 but the movement of these bodies is so extremely rapid that 

 it is impossible to determine their height with astronomical 

 precision. Still computations by Professor Herschel give a 

 height of about 118 miles*, and Professor Newcomb estimates 

 it to be about 100 miles")*. It is possible that a meteor w r ould 

 sometimes become inflamed by penetrating the atmosphere 



* Professor A. S. Herschel gives the height of twenty meteors varying 

 from 40 to 118 miles. l Nature/ vol. iv. p. 504. 



f Newcomb says : — " The lightning-like rapidity with which the meteors 

 darted through their course rendered it impossible to observe them with 

 astronomical precision ; but the general result was that they were first 

 seen at an average height of 75 miles and disappeared at a height of 55 

 miles. There was no positive evidence that any meteor commenced at a 

 height greater than 100 miles. These phenomena seem to indicate that 

 our atmosphere really extends to a height of between 100 and 110 miles. ' ? 

 i Popular Astronomy,' 1878, p. 389. 



