the Trigonometrical Operation . 
2 33 
SECTION SEVENTH. 
r 
An Account of the obfervations made during the courfe of the 
trigonometrical operation for', the determination of terrefrial re- 
fraSlion. Plate X, 
Article I. Preamble . 
. . N' ■ ■ ^ 1 - i ✓ 
ASTRONOMICAL refraction, or that which the rays of 
light fuffer in paffing from the heavenly bodies to our earth, 
hath, by the inveitigations of different philofophers, been nearly 
afcertained. From the theory of dioptrics, as well as expe- 
rience, it hath been proved, that the rays, in coming from a 
very rare into a very denfe medium, are gradually bent down- 
wards, out of their reCtilinear direction, into lines more or 
lefs curved in proportion to the angular diftance of the objeCts 
from the zenith, where obliquity ceafing, refraCtion ceafes 
likewife ; lince from that point light takes the Ihorteft route 
through the refraCting medium to the eye of the obferver* 
Hence it follows, that the apparent altitudes of celeftial ob- 
jects are greater than they otherwife would be by the quantity 
of this refraCtion, which is greateft at the horizon, amounting 
there to 33C 
The late Dr* Bradley from his experience has fhewn, that 
in the mean ftate of the barometer taken at 29.6 inches, and 
of Fahrenheit’s thermometer at 50°, the refraCtion at 45® 
of altitude is 57" (according to Dr. Maskelyne only 56 // I). 
Vol.LXXX. Hh In 
