JET 
ceiving a polish, and not soiling the fingers 
by the touch. It has sometimes been con- 
founded with this substance, but the dis- 
tinction between them is not difficult : cart- 
nel-coal wants the electrical properties of 
jet, and is much heavier. Magellan sup- 
posed, that jet was true amber, differing 
from the yellow kind only in the circum- 
stance of colour, and being lighter on ac- 
count of the greater quantity of bitumi- 
nous matter, which enters into its composi- 
tion. It emits, in combustion, a bitumi- 
nous smell ; it is never found in strata or 
continued masses, like fossil-stones, but al- 
wayk in separate unconnected heaps, like 
true amber. It is found in abundance in 
the Pyrennean mountains ; also in some 
parts of Portugal and Spain, in Sweden, 
Prussia, Germany, Italy, and Ireland. 
JET d’eau, a French term, frequently 
also used with us, for a fountain that casts 
up water to a considerable height in the 
air. A jet of water is thrown up by the 
weight of the column of water above its 
ajutage, or orifice, up to its source or reser- 
voir; and therefore it would rise to the 
same height as the head or reservoir, if cer- 
tain causes did not prevent it from rising 
quite so high. For first, the velocity of the 
lower particles of the jet being greater than 
that of the upper, the lower water strikes 
that which is next above it ; and as fluids 
press every way, by its impulse it widens, 
and consequently shortens the column. Se- 
condly, the water at the top of the jet does 
not immediately fall off, but forms a kind 
of ball or head, the weight of which de- 
presses the jet ; but if the jet be a little in- 
clined, or not quite upright, it will play 
higher, though it will not be quite so beau- 
tiful. Thirdly, the friction against the sides 
of the pipe and hole of the ajutage, will pre- 
vent the jet from rising quite so high, and a 
small one will be more impeded than a 
large one. And the fourth cause is the re- 
sistance of the air, which is proportional to 
the square of the velocity of the water 
nearly; and therefore the defect in the 
height will be nearly in the same propor- 
tion, which is also the same as the propor- 
tion of the heights of the reservoirs above 
the ajutage. Hence, and from experience, 
it is found that a jet, properly constructed, 
will rise to different heights, according to 
the height of the reservoir, as in the follow- 
ing table of the heights of reservoirs, and 
the heights of their corresponding jets; the 
former in feet, and the latter in feet, and 
tenths of a foot. 
JEW 
HEIGHTS OF RESERVOIRS AND THEIR 
JETS. 
Res. 
Jet. 
Res. 
Jet. 
Res. 
Jet. 
5 
4.9 
31 
28.3 
57 
49.0 
6 
5.9 
32 
29.2 
58 
49.7 
7 
6.8 
33 
30.0 
59 
50.5 
8 
7.8 
34 
30.8 
60 
51.2 
9 
8.7 
35 
31.6 
61 
52.0 
10 
9.7 
36 
32.5 
62 
52.7 
11 
10.6 
37 
33.3 
63 
53.5 
12 
11.6 
38 
34.1 
64 
54.2 
13 
12.5 
39 
34.9 
65 
54.9 
14 
13.4 
40 
35.7 
66 
55.7 
15 
14.3 
41 
36.6 
67 
56.4 
16 
15.2 
42 
37.4 
68 
57.1 
17 
16.1 
43 
38.1 
69 
57.8 
18 
17.0 
44 
38.9 
70 
58.6 
19 
17.9 
45 
39.8 
71 
59.3 
20 
18.8 
46 
40.5 
72 
60.0 
21 
19.7 
47 
41.3 
73 
60.7 
22 
20.6 
48 
42.1 
74 
61.4 
23 
21.5 
49 
42.9 
75 
62.1 
24 
22.3 
50 
43.7 
76 
62.8 
25 
23.2 
51 
44.4 
77 ' 
63.5 
26 
24.1 
52 
45.2 
78 
64.2 
27 
24.9 
53 
46.0 
79 
64.9 
28 
25.8 
54 
46.7 
80 
65.6 
29 
26.6 
55 
47.5 
30 
27.5 
56 
48.2 
JETSAM, any 1 thing thrown out of a 
ship, being in the danger of wreck, and by 
the vraves driven to the shore. See Flot- 
sam, 
JETTY head, a name given to that part 
of a wharf which projects beyond the rest, 
but more particularly the front of the 
wharf, whose side forms one of the cheeks 
of a wet or dry dock. 
JEWEL blocks, two small blocks which 
are suspended at the extremity of the main 
and fore-top sail yards, by means of an eye- 
bolt driven from without into the middle of 
the yard-arm parallel to the axis. The use 
of these blocks is to retain the upper part 
of the top-mast studding-sails beyond the 
sheets of the top sails, so that each of these 
sails may have its full force of action, which 
would be diminished by the encroachment 
of the other over its surface. 
JEWS, in church history, the descen- 
dants of Judah, the son of Jacob, and of 
the Israelites, commonly denominated the 
Twelve Tribes of Israel. This name was 
first given to those Jews who returned 
from the captivity of Babylon, because the 
tribe of Judah made the most conspicuous 
figure among them. 
