I 
LONGITUDE. 
dians on the parallel of that latitude, in 
miles. And, by this theorem, is the follow- 
ing table constructed. 
A TABLE, 
Shewing how many Miles answer to a De- 
gree of Longitude, at every Degree of 
Latitude. 
0 
Miles. 
i P 
Miles. 
P 
p 
Miles. 
p 
Miles. 
1 
59.99 
24 
54.81 
47 
40.92 
69 
21.50 
2 
59.97 
25 
54.38 
48 
40.15 
70 
20.52 
3 
59.92 
26 
53.93 
49 
39.36 
71 
19.54 
4 
59.86 
27 
53.46 
50 
38 57 
72 
18.55 
5 
59.77 
28 
52.97 
51 
37.76 
73 
17.54 
6 
59.67 
29 
52.47 
52 
36.94 
74 
16.53 
7 
59.56 
jO 
51.96 
53 
36.11 
75 
15.52 
8 
59.42 
31 
51.43 
54 
35.27 
76 
14.51 
9 
59.26 
32 
50.88 
55 
34.41 
77 
13.50 
10 
59.08 
33 
50.32 
56 
33.55 
78 
12.48 
11 
58.89 
34 
49.74 
57 
32.68 
79 
11.45 
12 
58.68 
3b 
49.15 
58 
31.79 
80 
10.42 
lo 
58.46 
36 
48.54 
59 
30.90 
81 
H.38 
14 
58,22 
37 
47.92 
60 
30.00 
82 
8.35 
15 
57.95 
38 
47.28 
61 
29.09 
83 
7.32 
16 
57.67 
39 
46.62 
62 
28.17 
84 
6.28 
17 
57.37 
40 
45.95 
63 
27,24 
85 
5.23 
18 
57.06 
41 
45.28 
64 
26.30 
86 
4.18 
19 
56.73 
42 
44.59 
65 
25.36 
87 
3.14 
20 
56.38 
4o 
43.88 
66 
24.41 
88 
2.09 
21 
56.0! 
44 
43.16 
67 
23.44 
39 
1 05 
22 
55. 6S 
45 
42.43 
68 
22.48 
90 
0.00 
23 
5.5.23 
46 
41.68 
Longitude, in navigation, the distance 
of a ship or place, east or west, from ano- 
ther, reckoned in degrees of tire equator. 
As the discovery of a method to find the 
longitude would render voyages safe and 
expeditious, and also preserve ships and the 
lives of men, the following rewards have 
been offered by act of parliament, as an en- 
couragement to any person who shall dis- 
cover a proper method for finding it out : 
the author or authors of any such method 
shall be entitled to the sum of 10,0001. if it 
determines the longitude to one degree of a 
great circle; to 15,0001. if it determines the 
same to two-thirds of that distance ; and to 
20,0001. if it determines the same to one- 
half of the same distance; and that half of 
the reward shall be due and paid when the 
commissioners of the navy, on the major 
part of them, agree that any such method 
extends to the security of ships within 80 
geographical miles of the shores, which are 
places of the greatest danger; and the 
other half, when a ship, by the appointment 
of the said commissioners, or the major part 
•f them, shall thereby actually sail over the 
VOL. IV. 
ocean, fi ora Great Britain to any such port 
in the West Indies as those commissioners, 
or tlie major part of them, shall choose for 
the experiment, without losing their longi- 
tude beyond the limits before-mentioned. 
The French, Dutch, Spaniards, and other 
nations, have likewise offered rewards for 
the same purpose. 
Since, by the motion of the earth round 
its axis, every point upon its surface de- 
scribes the circumference of a circle, or 
360°, in twenty-four hours time, it is plain 
it must describe 15° in one hour, because 
11° = 15. Hence the difference of longitude 
may be converted into time, by allowing one 
hourfor every 16 degrees, and proportionally 
for minutes ; also difference of time may be 
converted into difference of longitude by al- 
lowing 15“ for every hour, and proportionally 
for a greater or less time. Consequently by 
knowing the one we can easily find the other. 
Whatever contrivance, therefore, shows 
the hours of the day, at the same absolute 
point of time, in two different places, like- 
wise serves to find the difference of longi- 
tude between those places. Now, since an 
eclipse of the moon proceeds from nothing 
else but an interposition of the earth be- 
tween her and the sun, by which means 
she is prevented from reflecting the light 
she would otherwise receive from the sun, 
the moment that any part of her body be- 
gins to be deprived of the solar rays, it is 
visible to all those people who can see her 
at the same time ; whence, if two or more 
different people, at two or more different 
places, observe the times when it first be- 
gan or ended, or note the time when any 
number of digits was eclipsed, or when the 
shadow begins to cover or quit any remark- 
able spot, the difference of those times (if 
there be any) when compared together, will 
give the difference of longitude between 
the places of observation. 
The- longitudes of places may also be 
determined from the observations of solar 
eclipses, but these being encumbered with 
the considerations of parallaxes, are not 
near so proper as those of the moon ; and 
each of these happening but rarely, an- 
other excellent expedieht has been thought 
of, and that is the eclipses of Jupiter’s sa- 
tellites. 
Now as neither Jupiter nor any of his 
attendants have any native light of their 
own, but shine with a borrowed light from 
the sun, it happens that each of these, in 
every revolution about Jupiter, suffers two 
eclipses, one at their entrance into the sha- 
M 
