VOL. XXXIX. J FHILOSOPHICAL TRANSACTIONS. 105 



being of great service to them in removing the snow, in order to get at the 

 grass or moss underneath, which is their chief subsistence in Lapland. 



An Attempt to explain the Phenomenon of the Horizontal Moon appearing larger 

 than when elevated several Degrees above the Horizon, supported by an Experi- 

 ment. By Dr. Desaguliers. N° 444, p. SgO. 



This apparent increase of the moon's diameter, which a telescope with a mi- 

 crometer shows to be only apparent, is owing to the following early prejudice 

 we have imbibed from children. When we look at the sky towards the zenith, 

 we imagine it to be much nearer to us, than when we look at it towards the 

 horizon : so that it does not appear spherical, according to the vertical section 

 EFGHi, fig. 9, pi. 4, but elliptical, according to the section epghi. The sky 

 thus seen strikes the eye in the same manner as the long arched roof of the isle 

 of a cathedral church, or the ceiling of a long room. 



This being premised : let us consider the eye at c, on the surface of the 

 earth ; and imagine c at the surface to coincide with k at the centre ; to avoid 

 taking into consideration that the n)oon is really farther from the eye when in 

 the horizon, than when it is some degrees high. Now when the moon is at g, 

 we consider it as at g, not much farther than g ; but when it is at h, we 

 imagine it to be at h, almost as far again. Therefore, while it subtends the 

 nearly same angle as it did before, we imagine it to be so much larger, as the 

 distance seems to us to be increased. 



Dr. D. contrived the following experiment to illustrate this : he took two 

 candles of equal height and size, ab, cd, fig. 10 ; and having placed ab at the 

 distance of 6 or 8 feet from the eye, he placed cd at double that distance; then 

 causing any unprejudiced person to look at the candles, he asked which was 

 largest ? and the spectator said they were both of a size ; and that they appeared 

 so, because he allowed for the greater distance of cd ; and this also appeared to 

 him, when he looked through a small hole. Then desiring him to shut his 

 eyes for a time. Dr. D. took away the candle cd, and placed the candle ep close 

 by the candle ab, and though it was as short again as the others, and as little 

 again in diameter, the spectator, when he opened his eyes, thought he saw the 

 same candles as before. Whence it is to be concluded, that when an object is 

 thought to be twice as far from the eye as it was before, we think it to be twice 

 as large, though it subtends but the same angle. — And this is the case of the 

 moon, which appears to us as large again, when we suppose it as far again, 

 though it subtends only the same angle. 



The difference of distance of the moon in Perigeo and Apogeo, will account 



VOL. vm. P 



