THE SKY 57 



city. Soon even they darkened, the darkening spreading out 

 laterally from the centre, so that a vast semicircle of light only 

 remained, of parabolic shape. Even this light faded by degrees, 

 until the sky came to present only a rosy background, more and 

 more limited in extent, against which the sombre form of the 

 Arc de Triomphe stood out." 



The teacher will not fail to point out that in this description 

 the three striking phenomena of sunset, the glow, the horizon 

 colours, and the twilight arch, are expressly mentioned. The 

 appearance of horizontal clouds (stratus) at sunset is also, as is 

 explained below, a frequent phenomenon in clear weather. 



The facts connected with refraction at sunset, which have as 

 result the bringing of the sun up to the horizon when it has really 

 sunk below it, are too difficult to be dealt with in an elementary 

 course, but it is worth while to deliberately make the attempt to 

 prove that, owing to the enlargement of the horizon with increase 

 of altitude, the apparent time of sunset is not precisely the same 

 at the top of a hill as it is at the bottom. Encourage the 

 children to prove this for themselves in the holidays. Further, 

 especially if any members of the class go far north in the summer 

 season, they may very well try to prove to themselves that 

 sunset on the same evening becomes later as one travels westward. 

 In the latitude of Inverness, for example, a degree of longitude has 

 only a length of about thirty-seven miles. Roughly, then, sunset 

 is here a minute later for every nine miles to the westward. Two 

 boys at this distance from each other, and both furnished with 

 watches, could without great difficulty prove to themselves that 

 this difference exists, by noting the exact second of disappearance, 

 and later comparing their watches and results. During a holiday 

 in the Highlands of Scotland a clear view of the western sky is 

 easily obtained, and it may quite well happen that members of the 

 same family, on steamboat or motor-car excursions or in other ways, 

 may be ten or twelve miles distant from each other at sunset. 

 Even if the actual experiment cannot be made, it is well to suggest 

 the possibility of it, for we want above all to get the class to realise 

 that sunset is an appearance whose time of occurrence depends upon 

 the observer's elevation above the surface and upon his position on 

 it. It seems a sound principle to get at this fact so far as possible 



