JET D'EAU

, a French word, signifying a fountain that throws up water to some 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 reservoir; and therefore it would rise to the same height as the head or reservoir, if certain 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 impulfe it widens, and consequently shortens the column. Secondly, 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 depresses the Jet; but if the Jet be a little inclined, or not quite upright, it will play higher, though it will not be quite so beautiful. Thirdly, the friction against the sides of the pipe and hole of the ajutage, will prevent the Jet from rising quite so high, and a small one will be more impeded than a large one. And th<*> fourth cause is the resistance 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 proportion, which is also the same as the proportion 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 following 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.

Heights of Reservoirs and their Jets.
Res.	Jet.	Res.	Jet.	Res.	Jet.	Res.	Jet.
5	4.9	31	28.3	57	49.0	82	67.0
6	5.9	32	29.2	58	49.7	83	67.7
7	6.8	33	30.0	59	50.5	84	68.4
8	7.8	34	30.8	60	51.2	85	69.1
9	8.7	35	31.6	61	52.0	86	69.8
10	9.7	36	32.5	62	52.7	87	70.5
11	10.6	37	33.3	63	53.5	88	71.1
12	11.6	38	34.1	64	54.2	89	71.8
13	12.5	39	34.9	65	54.9	90	72.5
14	13.4	40	35.7	66	55.7	91	73.2
15	14.3	41	36.6	67	56.4	92	73.8
16	15.2	42	37.4	68	57.1	93	74.5
17	16.1	43	38.1	69	57.8	94	75.2
18	17.0	44	38.9	70	58.6	95	75.8
19	17.9	45	39.8	71	59.3	96	76.5
20	18.8	46	40.5	72	60.0	97	77.2
21	19.7	47	41.3	73	60.7	98	77.8
22	20.6	48	42.1	74	61.4	99	78.5
23	21.5	49	42.9	75	62.1	100	79.1
24	22.3	50	43.7	76	62.8	110	85.6
25	23.2	51	44.4	77	63.5	120	91.9
26	24.1	52	45.2	78	64.2	130	98.0
27	24.9	53	46.0	79	64.9	140	104
28	25.8	54	46.7	80	65.6	150	110
29	26.6	55	47.5	81	66.3	160	116
30	27.5	56	48.2

By various experiments that have been made by Mariotte, Desaguliers, and others, it has been found, that if the res<*>voir be 5 feet high, a conduct pipe 1 3/4 inch diameter will admit a hole in the ajutage from 1/4 to 3/<*> of an inch; and so on as in the following table:

Height Reservoi<*>	Diam. of the Ajutage.	Diam. of the Conduct Pipe.
5 feet	<*>/4 to 3/8 inch	1 3/4 inch
10	<*>/4 to 1/2	2
15	1/2	2 1/4
20	1/2	2 1/2
25	1/2	2 3/4
30	1/2 to 3/4	3 1/4
40	3/4	4 1/2
50	3/4	5 1/4
60	1	5 3/4 or 6
80	1 1/4	6 1/2 or 7
100	1 1/4 to 1 1/2	7 or 8

| But the size of the pipe will be more or lesa with the distance.

If it be required to keep any number of Jets of given dimensions playing, by one common conduct-pipe; the diameter of an ajutage must be found that shall be equal to all the small ones that are given, and from this its proper conduct-pipe. Thus, if there be 4. ajutages, each 3/4 of an inch diameter; then the square of 3/4 is 9/16, which multiplied by 4, the number of them, makes 36/16, the square root of which is 6/4 or 1 1/2, the diameter of an ajutage equal to all the other four; to which in the table answe<*>s a pipe of 8 inches diameter. In general, the diameter of the conduct-pipe should be about 6 times that of the ajutage.

See Mariotte's Mouvement des Eaux; Desaguliers's Exper. Philos. vol. 2, p. 127, &c; Clare's Motion of Fluids, p. 109; &c.