208 Observations to determine the amount of 
vation, in consequence of the freezing of the oil ; the frequent 
occurrence of snow drift ; and the haze which usually hangs 
near the horizon during a Polar winter, it must be admitted, 
that the repeating circle is not calculated, under such circum- 
stances, either for obtaining numerous observations, or for 
ensuring the degree of accuracy indispensibly requisite in 
observations for determining the amount of atmospherical 
refractions. 
Another method was suggested by Captain Kater, in April, 
1824, which is explained in the following words : 
“ Select a star which passes the zenith, and when this star 
“ and the Pole star are at the same altitude, take the distance 
“ between them by means of the repeating reflecting circle ; 
“ do the same when the star is in the zenith, and also when 
“ upon the meridian under the Pole. From the first observa- 
“ tions the true zenith distance of the stars may be readily 
“ obtained. By observations made when the star is in the 
“ zenith, the absolute refraction of the Pole star will be 
“ given, and from the observations made when the star is 
“ under the Pole, the refraction at that altitude can be easily 
“ deduced. Pursue the same method with other stars, care- 
“ fully marking at each observation the time and state of the 
“ barometer and thermometer. We shall thus be furnished with 
“ data, from which the refraction at the various altitudes can 
“ be computed with facility and accuracy." 
On considering, however, the difficulties already detailed 
in the use of the repeating circle, which rendered it impossible 
to take advantage of this ingenious suggestion of Captain 
Kater ; it occurred to Lieutenant Foster, that a more simple 
and accurate method of determining the amount of refraction, 
