208 ESSAYS BIOGRAPHICAL AND CHEMICAL 



with what is called an ' exhausted ' receiver, show that the 

 colour and motion of the electric spark vary in proportion 

 to the rarity of the air in the receiver. The more the air 

 is rarefied, the more movable and coloured is the electric 

 aura passing through it. On the contrary, the colour of 

 the spark approaches to whiteness, and moves with greater 

 difficulty, as the air is admitted. Will this observation 

 serve in any measure to account for the difference in 

 colour and motion of these electrical arches, for such I 

 presume to call them ? May we not suppose the more 

 coloured and brilliant portions of the aurora borealis to be 

 made in the rarer parts of the atmosphere, while the more 

 white and stationary ones possess the denser parts ? The 

 whitest arches which I saw were the most fixed/ 



Repeated attempts have been made to ascertain the 

 height of an aurora. Henry Cavendish, the celebrated 

 chemist, calculated the height from the data furnished by 

 three observers of an aurora which was seen in 1784 one 

 of whom was stationed at Cambridge, one at Kimbolton 

 in Huntingdonshire, and one at Blockley in Gloucester- 

 shire by simple trigonometry, knowing the angle which 

 the summit of the arch appeared to subtend with the 

 horizon in each case ; the results are very reasonably con- 

 cordant, and give an altitude of 52 to 71 miles. Many 

 subsequent attempts have been made, and with similar 

 results. One of the latest writers, Professor Birkeland, 

 of Christiania, gives as limits 62 to 124 miles (100-200 

 kilometres). The difficulty in such observations is to 

 make sure that the observers in different places have 

 been measuring the same arch at the same moment. I 

 shall have occasion later to bring evidence of a totally 

 different character to confirm the general accuracy of such 

 measurements. 



Between the years 1786 and 1793 John Dalton, who 

 was as indefatigable a meteorologist as he was a distin- 



