316 



Astronomical and Nautical Collections. 



may be obtained from the following rule, given in the first edition 

 of the requisite Tables. As 400 is to the difference of the thermo- 

 meter from 55°, so is the mean refraction to its correction, to be 

 subtracted, if the thermometer is higher than 55° ; but to be added, 

 if the thermometer is lower. As 300 is to the difference between 

 the altitude of the barometer and 30 inches, expressed in tenths of 

 an inch, so is the refraction corrected for the thermometer to the 

 correction on account of the barometer, which added to or sub- 

 tracted from the refraction corrected for the thermometer, accord- 

 ingly as the barometer is higher or lower than 30 inches, gives the 

 true refraction corrected on account of both. 



Example I. 



Taken from Mr. Lax's Tables, p. 278. 

 Barometer ... 28 '6 inches, 



Same as by Mr. Lax's method. 



95 



In this example 25" and 3" are the respective variations of the 

 sun and moon's refractions, and 4" the variation of the auxiliary 

 angle, all to be added, as the true refraction is less than the mean. 



