REFRACTION. 
on account of the barometer, and 400 : 450 
# : ; R the refraction corrected both on 
account of the barometer and thermometer ; 
which final corrected refraction is there- 
forg — 450 — jQ correct the 
11840 ^ 
same refraction, r, by means of the latter 
state, viz. barometer 30 and thermometer 
hv 
55, it will be as 30 : 5 : : r : R and 
^ „ 455 — 455^t 
400 : 455 — t : :R : K— 
hr the correct refraction. 
Mr. Simpson has determined, by theory, 
the astronomical reflections, from which he 
brings out this rule, viz. as 1 to .9986 or as 
radius to sine of 86° 58' SO ' , so is the sine 
of any given zenith distance, to the sine of 
an arc ; then ^ of the difference between 
this arc and the zenith distance, is the re- 
fraction sought for that zenith distance. 
And by this rule Mr. Simpson computed a 
table of the mean refractions, whicli are 
not much different from tiiose of Dr. Brad- 
ley and Mr. Mayer, and are as in the follow- 
ing table. See Simpson’s Dissertations. 
MR. SIMPSON’S TABLE OF MEAN RE- 
FRACTIONS. 
Appa- 
rent 
Alti- 
tude. 
Refrac- 
tion. 
Appa- 
rent 
Alti- 
tude. 
Refrac- 
tion. 
Appa 
rent 
Alti- 
tude. 
Refrac- 
tion. 
0° 
33' 0" 
17° 
2' 
50" 
38° 
1' 
7"' 
1 
23 50 
18 
2 
40 
40 
1 
2 
2 
17 43 
19 
2 
31 
42 
0 
58 
3 
13 44 
20 
2 
23 
44 
0 
54 
4 
11 5 
21 
2 
16 
46 
0 
50 
5 
9 10 
22 
2 
9 
48 
0 
47 
6 
7 49 
23 
2 
3 
50 
0 
44 
7 
6 48 
24 
1 
67 
52 
0 
41 
8 
5 59 
25 
1 
52 
54 
0 
38 
9 
5 21 
26 
1 
47 
56 
0 
35 
10 
4 60 
27 
1 
42 
58 
0 
32 
11 
4 24 
28 
1 
38 
60 
0 
30 
12 
4 2 
29 
1 
34 
65 
0 
24 
13 
3 43 
30 
1 
30 
70 
0 
19 
14 
3 27 
32 
1 
23 
75 
0 
14 
15 
3 13 
34 
1 
17 
80 
0 
9 
16 
3 1 
36 
1 
12 
85 
0 
It is evident that all observed altitudes of 
the heavenly bodies ought to be diminished 
by the numbers taken out of the foregoing 
table. It is also evident that the refraction 
diminishes the right and oblique ascensions 
of a star, and increases the descensions ; it 
increases the northern declination and lati- 
tude, but decreases the southern ; in the 
eastern part of the heavens it diminishes 
the longitude of a star, but in the western 
parts of the heavens it increases the same. 
See Quadrant. 
Refraction, terrestrial, is that by which 
terrestrial objects appear to be raised higher 
than they really are, in observing their alti- 
tudes. The quantity of this refraction is 
estimated by Dr. Maskelyne at one-tenth 
of the distance of the object observed, ex- 
pressed in degrees of a great circle. So, if 
the distance be 10,000 fathoms, its tenth 
part 1000 fathoms, is the sixtieth part of a 
degree of a great circle on the earth, or 1" 
which therefore is the refraction in the alti- 
tude of the object at that distance. But 
M. Le Gendre is induced, he says, by seve- 
ral experiments, to allow only one-fourteenth 
part of the distance for the refraction in al- 
titude. So that, upon the distance of 
10,000 fathoms, the fourteenth part of 
which is 714 fathoms, he allows only 44" 
of terrestrial refraction, so many being con- 
tained in the 714 fathoms. See his Memoir 
concerning the trigonometrical operations, 
&c. Again, M. de Lambre, an ingenious 
French astronomer, makes the quantity of 
the terrestrial refraction to be the eleventh 
part of the arch of distance. But the Eng- 
lish measurers. Col. Edward Williams, Capt. 
Mudge, and Mr. Dalby, from a multitude 
of exact observations made by them, deter- 
mine the quantity of the medium refraction 
to be the twelfth part of the said distance. 
The quantity of this refraction, however, 
is found to vary considerably, with the dif- 
ferent states of the weather and atmosphere, 
from the fifteenth part of the distance to the 
ninth part of the same, the medium of which 
is the twelfth part, as above mentioned. 
Some whimsical effects of this refraction are 
also related, arising from peculiar situations 
and circumstances. Thus, it is said, any 
person standing by the side of the river 
Thames, at Greenwich, when it is high wa- 
ter there, he can see the cattle grazing on 
the Isle of Dogs, which is the marshy mea- 
dow on the other side of the river at that 
place; but when it is low water there, he 
cannot see any thing of them, as they are 
hid from his view by the land wall or bank 
on the other side, which is raised higher than 
the marsh, to keep out the waters of the 
river. This curious effect is probably owing 
to the moist and dense vapours, just above 
and rising from the surface of the water, 
being raised higher or lifted up with the 
surface of the water at the time of high 
tide, through which the rays pass, and are 
the more refracted. 
