448 0?i Refraction* 



found that his correction for the thermometer was a littid 

 over- rated ; and accordingly, for his new table, altered it 

 to Vr * or eacu degree. And here it may be observed that 

 La Caille did not correct his altitudes above 36° at Paris, 

 and 30° at the Cape ; first, because he only noted the baro- 

 meter and thermometer in the night, when he observed 

 stars below 30° of altitude. Secondly, because, that at 36 } 

 of altitude, where the mean refraction is about !•£■ minute, 

 the variation which belongs to 10 degrees or' the thermo- 

 meter only amounts to 3£ seconds ; a quantity about equal 

 to the limits of the errors of observations made with an in- 

 strument of six feet. 



The formula given byEuler* appeared also about this 

 time. It took into account the variation of the refraction 

 depending upon the thermometer and barometer, but was 

 certainly too complicated to be generally adopted. He shows 

 however, that in very different hypotheses the refraction 

 will be sufficiently exact, if taken in the inverse ratio of the 

 degrees of heat, when the star or planet is not too near the 

 horizon, but the precise quantity of this ratio was unknown 

 to him. 



In this state the refraction stood when Dr. Bradley took 

 the subject into consideration, and began to find its quantity 

 from his own observations. The rule which he adopted, 

 although a very elegant one, he neither lived to complete 

 nor to present to the world ; but it was published after his 

 death by Dr. JMaskelyne f, and has commonly been used 

 in England up to the present time. He found the mean re- 

 fraction at 45° of altitude 57", and, that at all other alti- 

 tudes it was equal to 57" multiplied by the tangent of the 

 zenith distance, diminished by three times the refraction. 

 Then supposing the mean state of the atmosphere to be at 

 29*6 in. of the barometer, and 50° of Fahrenheit's thermo- 

 meter, he made the true or corrected refraction equal to 



57 " x ,,(Z.D.~Sr) x b ^: x -ggl. where it is to be 



understood that the mass of air is supposed to increase in 

 bulk T J- - for each degree of Fahrenheit's scale. 



A variety of experiments have been made at various 

 times to ascertain the increase in bulk of a quantity of air 

 represented by unity for a certain number of degrees of rise 

 of the thermometer. The following is a list of some of 

 them J : — 



* Mnj. de l'Ac dc Berlin, 1754, p. 131. 



f Pief. to 1st vol. of Obs. 1765. Phil. Tram. 1764 and 17S7, p. 157. 

 Req. Tables, &c. 



f Sec La Landes Astr. 2241. 3d ed. Thomson's Chemistry, vol. i. p. 489. 

 La Place's Mec. Ce!. vol. iv. p. 270. Phil. Trana. 1809, &c. &c. 



