INTRODUCTION'. 57 



II. ASTRONOMICAL OBSERVATIONS 



TO DETERMINE THE HORIZONTAL REFRACTION. 



The inquiries into refraction are, it is true, pro- 

 perly of such a nature, tliat they appear to be 

 more accurate and more dehcate observations than 

 seamen are generally supposed to make. Two 

 advantages, however, concur here, which may ren- 

 der the endeavours of the seaman useful to this 

 important element of practical astronomy. On 

 the one hand the effects of refraction are precisely 

 the strongest in those observations, for which the 

 seaman has the best opportunity, and requires such 

 simple arrangements ; and, on the other hand, his 

 chief instrument, the reflecting sextant, both by the 

 excellence of tlie theoretical contrivance, and the 

 perfection in which it is now manufactured by 

 good artists, is peculiarly adapted to measure even 

 small magnitudes with sufficient accuracy, as is 

 proved by the lunar distances and the measurement 

 of the sun's diameter. The increase of refraction 

 by cold affords an additional motive for examining 

 its effects on a voyage to the north pole. 



The chief observations which the navigator may 

 make on the effects of refraction, are enumerated 

 in the following instructions : 



Let him accurately observe the true time at 

 which the upper and lower limb of the sun, at his 

 rising or setting, touch and quit the horizon, and 



