150 TERRESTRIAL REFRACTION. SECT. XVIII. 



below the pole was noticed by Alhazen, a Saracen 

 astronomer of Spain, in' the ninth century, but its exis- 

 tence, was known to Ptolemy in the second, though he 

 was ignorant of its quantity. 



The refraction of a terrestrial object is estimated dif- 

 ferently from that of a celestial body. It is measured 

 by the angle contained between the tangent to the 

 curvilineal path of the ray where it meets the eye, and 

 the straight line joining the eye and the object (N. 187). 

 Near the earth's surface the path of the ray may be 

 supposed to be circular ; and the angle at the center of 

 the earth corresponding to this path is called the hori- 

 zontal angle. The quantity of terrestrial refraction is 

 obtained by measuring contemporaneously the elevation 

 of the top of a mountain above a point in the plain at its 

 base, and the depression of that point below the top of 

 the mountain. The distance between these two sta- 

 tions is the chord of the horizontal angle ; and it is easy 

 to prove that double the refraction is equal to the 

 horizontal angle, increased by the difference between 

 the apparent elevation and 4he apparent depression. 

 Whence it appears that in the mean state of the atmos- 

 phere, the refraction is about the fourteenth part of the 

 horizontal angle. 



Some very singular appearances occur from the acci- 

 dental expansion or condensation of the strata of the 

 atmosphere contiguous to the surface of the earth, by 

 which distant objects, instead of being elevated, are de- 

 pressed. Sometimes being at once both elevated and 

 depressed they appear double, one of the images being 

 direct, and the other inverted.. In consequence of the 

 upper edges of the sun and moon being less refracted 

 than the lower, they often appear to be oval when near 

 the horizon. The looming also or elevation of coasts, 

 mountains, and ships, when viewed across the sea, 

 arises from unusual refraction. A friend of the au- 

 thor, while standing on the plains of Hindostan, saw 

 the whole upper chain of the Himalaya mountains start 

 into view, from a sudden change in the density of the 

 air, occasioned by a heavy shower after a very long 

 course of dry and hot weather. Single and double im- 

 ages of objects at sea, arising from sudden changes of 



