Encyclopaedia Britannica > Volume 7, ETM-GOA

Sea. II. G E O G
Principle* the dimenfions of the whole. An account of the moft
and remarkable attempts for difcovering the circumference
. ra :ce' of the globe has been given in the preceding fe&ion.
The foundation of the whole is to obtain an exadl
meafure of one degree of the meridian ; which being
once got, we have only to multiply the number of
miles, feet, or any other meafure employed, by 360,
the number of degrees in the circumference, and the
produft is that of the whole globe. This being ob¬
tained, we may eafily determine its fuperficial and fo-
lid contents by the geometrical methods employed in
other cafes. According to the beft calculations which
have yet appeared, the dimenfions of this globe are as
follow.
Englifh miles.
One minute of a degree contains 1/^
A degree - - 694-
The circumference - - 24,930
The diameter - - 7935f
The femidiameter - 39fi7x
The fuperficial meafure 200,000,000
The folid contents two hundred and fixty-fix thou-
fand millions of cubic miles.
A fecond of a degree is no more than lOi^-Eng-
lifii feet.
In making meafurements of this kind, the principal
difficulty arifes from the want of an abfolutely level
furface, the length of which may be determined by
adlual menfuration as the foundation of our calcula¬
tions. Snellius, as has already been mentioned, had a
Angular opportunity of this kind by means of a great
extent of ice ; and fimilar conveniences might be had
on the frozen lakes in the north of Europe, though,
difficulties would there arife from the great refraction
of the atmofphere. It mull likewife be confidered*
56 that there is always fome difference between the ap.
©ifference parent level and the true, which in great diftances is
thelippa- aPt to a^e<^ our ca^ulatIons materially. A truly,
rent and level furface is the fegment of any fpherical furface
true levels, concentric to the furface of the earth : thus the fur¬
face of the fea or any large piece of water when at reft
forms itfelf into a true level. A true line of level then
is an arc of a great circle, which we fuppofe to be de-
fcribed upon a truly level furface. The apparent le¬
vel is a ftraight line drawn tangent to the true level;
whence every point of the apparent level, excepting
only that of contaft, is fomewhat higher than the true
level. This difference is eafily known after the femi¬
diameter of the earth is known. Thus in fig. 6. let
the obferver Handing at A look through a tdefcope
placed horizontally at the objeft B; here BAG is a,
right-angled triangle, in which if AC be made radius,
AB will be tangent, and CB fecant of the angle ACB.
Now, to find this tangent, fay, as the number of feet
in AC is to the number of feet in AB, the diftance of
the objedt; fo is AC as radius to A B as tangent.
Then having found the tangent AB in the table, we
have the fecant CB*, from which if the radius CG be
taken, the remainder GB is the excefs of the fecant
above the radius, or the height of the apparent level
above the true. The following table was eonftrudted
by Q&ffini.
R A P H Y.
645
A Table flowing the Height of the Apparent Lewi above Pra'"£ple*
Plate
CCXI.
13
I +
16
17
Feet.
XOI
202
3°4
406
507
609
710
8l2
914
IOJJ
HI?
I2l8
132O
I42 I
'I'3
I625
1726
’a 28
| 19 O !929
; 20 ® 2031
I 2 r § 2132
j 22 § 2234
e -3 § ’336
! 24 -5 2437
. 25 S 2539
| 26 ^ 2640
27 - 2742
u ~ 28«
^ 29 3 2945
g 30 -g 3047
S 3i *o 3148
5 32 3 3250
1*33 3351
-o 34 “ 3453
^ 35 c 3554
^ 36 ^ 3656
g 3757
1 38?9
' 37
! 38 re _
39 g 3961
40 ^ 4062
41 .j 4164
- - 4265
4367
4468
4570
4672
4773
4875
4976
5078
5179
5281
5383
5484
5586
5687
5789
5890
5992
6094
•*3 40
: 42
* 43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
Inch.
6.8
1.6
8.4
3 2
10.0
4.8
11.6
6.4
1.2
8.0
2.8
9*6
4.4
11.2
6.0
0.8
7.6
2.4
9.-2
4.0 U
10,8 =3
5-6 -1
0.4 o
7.2 i
3*6 §
io-4
5.2 *£
0.0 C
. 1.6 £
8.4 S,
3.2 «
10.0
4.8 C
11.6
6.4
12
8-o t;
2.8 -s
9.6
44
11.2
6,0
0.8
7.6
2.4
9.2
4.0
10.8
5.6
0.4
7.2
2.0
8.8
36
10.4
5-2
0.0
0.074
0.296
2.670
bO
4-746
7.405
2fo*
10.68©