Entretiens familiers sur l'astronomie, avec des chapitres sur la géographie et l'navigaton . e A E représente le rationnel ou horizon vrai,et la ligne D, une tangente à l'méplatdu côté de masses C, l'horizon visible ou sensible. Whenwe maintenant mesurer l'altitude de S, le soleil, nous ame-po ur l'angle triste ; mais l'altitude nous re-quire est l'angle S A E ; c'est pourquoi nous mesurons toomuch, par l'angle E UN D. notre horizon a été en deçà de l'horizon rationnel par theangle E A D, et cet angle est appelé le dip.Nous voyons donc que le dip doit être subtractedfrom observé l'altitude ; et que son amountdepends entièrement sur la hauteur

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Familiar talks on astronomy, with chapters on geography and navigaton . e A E represents the rational or true horizon, and the line A D, tangent to the earths surfaceat C, the visible or sensible horizon. Now whenwe measure the altitude of S, the sun, we meas-ure the angle SAD; but the altitude we re-quire is the angle S A E; hence we measure toomuch, by the angle E A D. Our horizon has dipped below the rational horizon by theangle E A D, and this angle is called the dip.We see, then, that the dip must be subtractedfrom the observed altitude; and that its amountdepends entirely upon the height of the ob-server. Tables are inserted in our books onnavigation giving the dip corresponding toheights up to one hundred feet. The angle Dip of the Horizon. 221 E A D is small even up to one hundred feet, being for that height but 9/ 51. Navigatorson the decks of their vessels are rarely overtwenty-five feet above the level of the sea. I must call to your attention that the heightA B, in the diagram, is very much exaggerated.All diagrams of the kind are. They are only. Fig. 12. designed to illustrate. If I should attempt todraw it on a scale, the angle E A D, represent-ing the dip, even for one hundred feet, wouldbe less than a pencil-mark. The distance O Bbeing the radius of the earth, or four thousandmiles, and the distance A B only fifty or onehundred feet. The correction for refraction is the most vex-ing of all the corrections, because it depends 222 Familiar Talks on Astronomy•, etc. for its value upon the earths atmosphere, whichis constantly changing in density. When a rayof light passes obliquely from one medium toanother of different density it is bent, or re-fracted, out of its course. This is best seen byplacing a stick obliquely in water. Now theearths atmosphere is not of uniform density;hence a ray of light from the sun, or othercelestial body, upon entering it is bent moreand more downwards, describing a curve, untilit enters the eye of the observer. The observ

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