450 On Refraction* 



progression, -therefore an hypothesis which participates of 

 both these progressions would seem to represent the re- 

 fraction, and the observed diminution of heat in the at- 

 mospheric layers ; accordingly he makes this curious as- 

 sumption, and deduces a formula by means of it. 



He then applies the same analysis to the finding of an 

 equation for the retraction at altitudes below 12% and gives 

 us an expression which has the advantage of being inde- 

 pendent of all hypotheses respecting the constitution of 

 the atmosphere, resting only upon the nature of it in the 

 place where the observation is made, as indicated by the 

 barometer and thermometer, after which he determines the 

 value of his coefficients. With respect to that depending 

 upon the thermometer, he requested M. Gay Lussac to re- 

 peat his experiments, with all possible care, by graduating 

 his thermometers exactly, and by paying the gteatest atten- 

 tion to dry the tubes well, which he made use of: for it 

 appeared to him, that upon this depended principally the 

 great differences in the re suit 9 hitherto found by philoso- 

 phers. Attending well to the expansion of glass*, and to 

 the corrections on account of the variableness of the baro- 

 meter, during each experiment Gay- Lussac found, by a mean 

 of twenty-five experiments, that a volume of air expressed 

 by unity at zero of temperature of the centigrade thermo- 

 meter became 1*37.5 at the heat of boiling water, under a 

 pressure equivalent to that of a column of mercury = 0*76 

 of a metre in height. 



The other coefficient was determined by M. Delambre, 

 who, by comparing a great number of observations, found the 

 refraction to be 186'"f28at 50'of apparent altitudef,thetem- 

 perature being zero, and height of the barometer 0* 76 metre. 



After this he proceeds to consider the effect of moisture 

 in the atmosphere, and concludes the following values for 

 the increase of the refraction, for extreme humidity in the 

 air from 15' to 45- of temperature. 



Degree--. Increase of Refraction. 



15° 0-.563" t, 



20 0-/14 /, 



25 O977 t, 



30 1274 t, 



35 1-051 t, 



40 . .2-122 t y 



Where represents the apparent altitude. 



* Upon this and some other points connected with the subject of this 

 •viper, see Thomson's Chemistry, hook i, cliv. '.?, seel. 4, edit, of 18 iO. 



'• According to the new centesimal division of the circle; hut this, ac- 

 ^ to our division of the circle, will be 60" 499872 at 45° of apparent 



-, when the barometer is 29 92152 EugL in., «:ud Fahrenheit's theun. 



cord*. 

 altitnj* 



