which hitherto has been jinohserved hy Philosophers. 195 



uould be refracted down to our feet; and therefore the sun's rays 

 could not occupy a greater space upon the retina after sunset than 

 they did before, and that is no larger than the angle of his dia- 

 meter — half a degree. 



Those who argue that the sun's diverging rays, after he has 

 set, and the atmosphere, in consequence of increased cold, be- 

 come denser, are reflected into the eye from all parts of the hea- 

 vens, by the particles of air, are bound to show in what manner 

 the rays of blue colour are brought into the eye ; for, if the blue 

 colour of the heavens be owing to the particles of air reflecting 

 the blue rays in greater abundance than all the rest, an increased 

 quantity of light would only produce a more intense blue ; and 

 there is no other way in which the twilight can be produced, but 

 in the manner I have supposed. 



In the summer the sun lingers a long while on the horizon 

 before he sets, and the twilight is very full; whereas in the win- 

 ter he sets almost immediately, and the twilight is hardly per- 

 ceptible ; and as the twilight is owing to the density of the at- 

 mosphere, and this density is increased by cold, it will perhaps 

 occur to the reader to demand why the twilight is not stronger 

 in winter than in summer. To this I answer. That in the sum- 

 mer the sun is very far to the northward, and after he has set, his 

 Kght, to come to us, passes in great measure through that part 

 of the atmosphere which is densest and refracts light the most; 

 and accordingly, we find that the twilight is always stronger on 

 the north than the west side of him; and while it extends but a 

 very little way towards the west, it stretches over the whole of 

 the north part of the horizon. In winter, on the contrary, the 

 sun's rays at setting, in order to come to us, must pass through 

 the torrid zone, where the atmosphere is rarest and refracts light 

 the least ; and not only this, but immediately after he has set, 

 his ray?, to come to us, must travel a hundred degrees of latitude 

 further than they do in the summer; and that part of the twilight 

 which appears to us to be but just above the horizon, is very near 

 the zenith to those who are in the same degree of south latitude, 

 and consequently is almost too faint to be visible. 



6. I have never had the good fortune to witness any of the 

 phsenomeua of the aurora horealis, and the idea my imagination 

 has formed of it from description may be very different to what it 

 is in reality. From all accounts however it appears, in the ground- 

 work at least, to bear a very close resemblance to the twilight, 

 80 much so that it is sometimes called the northern twilight; 

 and if I could but satisfactorily account for the crackling noise 

 that is sometimes heard with it, I should pronounce it to be no 

 other than a twilight attended with some additional phaenomena 



N 2 wliicU 



