'314 WELLS'S NATURAL PHILOSOPHY. 



aim at a point apparently below it, otherwise the weapon will miss, by pass- 

 ing too high. 



A river, or any clear water viewed obUquely from the bank, appears more 

 shallow than it really is, since the hgbt proceeding from the objects at tho 

 bottom, is refracted as it emerges from the surface of the water. Tho depth 

 of water, under such circumstances, is about one third more than it appears, 

 and owing to this optical deception, persons in bathing are liable to get be- 

 yond their depth. 



Lio;ht, on enterinor tbe atmosphere, is re- 



Whatisatmos- /. , . -, , . 



pheric refrac- iractecl Id a greater or less degree, m propor- 

 tion to the density of the air ; consequently, 

 as that portion of the atmosphere nearest the surface 

 of the earth possesses the greatest density, it must also 

 possess the greatest refractive power. 



_,, ^ _ , , From this cause the sun and other celestial bodies are never 



Wliat effect has 



refraction upon seen in their true situations, unless they happen to be verti- 



tii" J"u v'°" °^ ^^^ ' ^^^ ^^^^ nearer they are to the horizon, the greater will 

 be the influence of refraction in altering the apparent place of 

 any of these luminaries. 



This forms one of the sources of error to be allowed for in all astronomical 

 observations, and tables are calculated for finding the amount of refraction, 

 depending on the apparent altitude of the object, and the state of the barom- 

 eter and thermometer. When the object is v6rtical, or nearly so, this error 

 is hardly sensible, but increases rapidly as it approaches the horizon ; so that, 

 in the morning, the sun is rendered visible before he has actually risen, and 

 in the evening, after ho has set. 



For the same reason, morning does not occur at the in- 

 what 13 the gtant of the sun's appearance above the horizon, or night 

 light? set in as soon as he has disappeared below it. But both 



at morning and evening, the rays proceeding from the sun 

 below the horizon are, in consequence of atmospheric refraction, bent 

 down to the surface of the earth, and thus, in connection with a reflect- 

 ing action of the particles of the air, produce a lengthening of the day, termed 

 twilight. 



1 hat man- ^^ ^'''^ density of the air diminishes gradually upward from 



ner is lii^ht re- the earth, atmospheric refraction is not a sudden change of 

 ataw^phere?*'^ direction, as in the case of the passage of light from air into 

 water, but the ray of light actually describes a curve, being 

 refracted more and more at each step of its progresa This applies to 

 the light received from a distant object on the surface of the earth, which is 

 lower or higher than the eye, as well as to that received from a celestial ob- 

 ject, since it must pass through air constantly increasing or diminishing in 

 density. Hence, in the engineermg operation of leveling, this refraction must 

 bo taken into consideration. 



