which hitherto has been unolserved ly Philosophers. 197 



any refracting power in the atmosphere, but because, by dimi- 

 nishing the velocity of light in a greater degree, it increases the 

 power of the earth's attraction. 



If we admit this hypothesis, and it is the only one that will 

 account for the refraction of the atmosphere, the crackling of the 

 aurora iGreolU is explained in the rays of light forcing their way 

 against the resistance of the atmosphere, and the crackling of 

 electricity may be owing to the same cause. If it be objected 

 that, as light is cons^tantly passing through the air, and the air as 

 constantly resisting it, this noise ought to be heard at all times ; 

 I reply by referring to another sense, the sense of smelling. In 

 all warm climates there is a regular land and sea breeze: the 

 wind blows constantly upon the land during the greatest part 

 of the night ; but a little before the dawn the land breeze springs 

 up, and brings wiih it all the scents that the earth exhales as far 

 as it extends, which i have sometimes known, on the coast of 

 Sumatra, to be more than forty miles. The scent of the land is 

 clearly perceived by every person who happens to be upon deck 

 the moment this breeze reaches him : but it lasts only for a mo- 

 n^ent ; for as soon as the sense of smelling is accustomed to these 

 effluvia which the wind brings to it, the effect is destroyed, and 

 it can only be excited by something that is more powerful. A 

 person on first going into a flower garden, may be almost over- 

 come with a delicious scent ; and yet in a few minutes he will only 

 be able to perceive it by going close to the flowers from whence 

 it proceeds. The scent of the garden is as strong as ever ; but 

 after the mind is accustomed to it, it requires a stronger excite- 

 ment to be able to perceive it : and in like manner the faint noise 

 that tl'.e light may be always making in passing through the air is 

 so uniform, that the mind is incapable of perceiving it: but wrhen 

 a body of light very suddenly forces its way through the atmo- 

 sphere, within perhaps a few feet of us, where there was none be- 

 fore, there is a new incitement, and totally different from the 

 uniformity that prevails every whore else. 



The aurora lorealis, by the descriptions I have read of it, is 

 a faint light, like the twilight, stroivgest at the horizon, and fading 

 away gradually as it rises towards the zenith : it is always strong- 

 est in that part of the honzon which is in a line with the sun ; 

 and so far their relationship may be traced in their resemblance, 

 and the phaenomena be attributed to the same cause. The aurora 

 boreulis however is frequently in motion, while the twilight is al- 

 ways still ; and it is only by assigning a suificient cause for this 

 phaenomcuon that I can have any pretensions of establishing my 

 hypothesis. 



It was the custom in Lord Collingwood's fleet off Cadiz to set 

 th« bearings of the high land over that town every morning at 



N3 day 



