CELESTIAL PHJENOMENA 



ground, and in the lower part of the atmosphere, I could 

 detect no changes of temperature or moisture by the me- 

 teorological instruments, and even though small stars of 

 the fifth and sixth magnitude appeared still to shine with 

 undiminished and equable light, condensations were tak- 

 ing place in the higher regions of the atmosphere, which 

 modified the transparency of the air, or rather its reflecting 

 power,*in some peculiar and to us unknown manner ? The as- 

 sumption of such meteorological processes near the limits 

 of our atmosphere is favoured by observations made by 

 the acute Olbers ("), "of sudden flashings and pulsations 

 which, in the course of a few seconds, vibrate throughout 

 the whole of a comet's tail, which is seen at the same time 

 to lengthen several degrees, and again to contract. As the 

 different portions of a comet's tail, which is millions of 

 miles in length, are at very unequal distances from the 

 Earth, it is not possible, according to the laws of the velocity 

 and propagaiton of light, that actual alterations in a cosmical 

 body filling so immense a space, should be seen by us to 

 take place in such short intervals of time." These considera- 

 tions by no means exclude the reality of variable emana- 

 tions around the denser envelopes of the nucleus of a comet, 

 or of sudden brightenings of the Zodiacal light from internal 

 molecular movements, or of changes due to variations in 

 the light reflected by the nebulous matter of which the ring 

 is composed ; but they should make us careful to distin- 

 guish between effects which should be referred to the cos- 

 mical ether and to the regions of space, and those which are 

 referrible to the terrestrial atmospheric strata through which 

 the bodies existing in space are beheld by us. There are 

 well observed facts, which shew us that we are not able to 



