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Rh for all sorts of purposes, including demolitions, and when the Allied advance began in August one of their most important duties was the removal of enemy land mines and booby traps.

Advances in Technique. Probably the most important of these was in the development of " listening apparatus." In the fortress mining systems of loo'years ago an essential feature was the provision of listening galleries pushed out at regular intervals from the front lines of permanent defence galleries; but judgment of direction and distance of the enemy's workings depended en- tirely on the trained ear of the listener, which might easily be de- ceived. In the World War listening instruments were invented and employed for the first time.

After much experiment two types were approved, the geo- phone and the seismomicrophone.

The Geophone is on the principle of the stethoscope. A flat circular wooden frame contains mercury enclosed between two discs of mica. Two nipples on the edge of the frame provide for the attachment of ear-pieces with rubber tubes. Sound can be heard through the geo- phone about two and a half times as loudly as with the unassisteo" ear. A single geophone can do no more than magnify sound ; for this purpose, however, it is very valuable, as it not only increases the distance at which enemy workings can be heard, but it helps the listener to distinguish between various sounds, such as picking, shovelling, talking, etc. With two geophones direction can be ob- tained, as follows : the two are placed at either end of a frame about 18 in. long, with a compass between them. To ascertain the direction horizontally the frame is placed on the ground and turned about un- til the sound is equally audible in both ears. The source of sound will then be in a direction at right angles to the line joining the two geophones, and can be plotted from the compass. If a bearing is then taken from another point, the intersection of the two will give the vertical line in which the source must be. To get the direction up or down, the geophones must be placed against the wall of the gallery. An example of the value of this method is given by Standish Ball :

" A portion of our trenches had been captured by the enemy and heavily wired to repel our expected counter-attack. This was due to take place on the 27th of the month, and the miners were asked to assist in the enterprise and destroy the hostile wire. Owing to the short time available a shaft had to be sunk from the end of a sap, and the objective was reached on the i6th, just as the enemy were heard starting their mining operations. A geophone observa- tion was immediately made, followed by further ones during the ensuing week. On the 24th the enemy were heard talking and laughing, without the aid of an instrument, only 6 ft. away, and to all intents and purposes on the point of breaking through. On examination of the previous observation, however, it was decided that the direction of his gallery was approximately the same as our own, and that he would probably pass along parallel to it. This is what actually happened, and the mine was blown successfully on the appointed day. If it had not been for careful and accurate listening the mine would have been blown prematurely and the success of the infantry attack jeopardized."

The Seismomicrophone, an electric detector, does not give direc- tion, but saves man-power. " As many as 50 galleries might be connected up to the switch-board of a central listening chamber, situated in some quiet position behind the mining system. In the event of sounds being heard from the detector in any particular gallery a listener was immediately dispatched with geophones to investigate." This economy of listeners was very important, es- pecially as the end of a gallery is often a very dangerous place.

The following table gives (in feet) the distance at which sounds can be heard by the various methods in chalk and clay :

Average Chalk

Nature of sound

Naked ear

Seismomicrophone

Geophone

Picking Shovelling Talking

125

70

12

175 70

45

250

I2O

50

Average Clay

Picking Shovelling Talking

50

8

5

70 15

10

I2O

30 15

Mine Rescue Work. One of the greatest dangers in mining has always been the presence in galleries of carbon-monoxide gas from the exploded mines. Owing to the quantity of mines blown and the large charges used this danger became very serious in 1915. " In six weeks one tunnelling company had 16 killed, 48 sent to hospital, and 86 minor cases treated at the shaft head and returned to the company billets." To meet this danger an instructor in the use of Proto apparatus was sent from England in June 1915, and schools for the teaching of rescue work were started.

The Proto apparatus consists roughly of twin cylinders of com- pressed oxygen, worn slung over the back, and supplying oxygen to a breathing bag containing caustic soda which absorbs COz from

the air exhaled. As this apparatus is in use in civil life it is not neces- sary to describe it here. The Proto set contains oxygen enough to last two hours. The Salvus apparatus is a lighter and more portable modification of the Proto, and is good for about half an hour. Oxygen resuscitating apparatus were also used. Apparatus special to the war were the mine stretcher, designed for dragging a man along a gallery and lifting him up a shaft, and a mine gas-testing set designed in the central laboratory at General Headquarters.

An officer of the R.A.M.C. having been attached to the staff of the engineer-in-chief for mine rescue work, the pathological aspect of the question was very thoroughly investigated.

Bored _Mines. These do not appear to have been much used in their original intention that of pushing a thin pipe towards an enemy gallery, passing a small charge through it to blow a camouflet, and then filling the latter with a charge of 300 or 400 Ib. of explosive. Larger bored mines with tubes up to 6 in. diameter were, however, used to some extent to make connexions. In some cases trenches were made in this way 14 ft. deep and 30 ft. wide. Trenches were also made by sinking bore holes from the surface and placing charges in them 4. ft. deep and 6 ft. apart. This method gave good cover in a very short time.

See The Work of the Royal Engineers in the European War 1914-19, section " Military Mining " (1921), which has been freely quoted in this article; also The Work of the Miner on the Western Front, 1915-18, by H. Standish Ball (Paper read at a meeting of the Inst. of Mining and Metallurgy, April 10 1919). The mine warfare on the western front is discussed from the German point of view in Schwarte, Militarischze Lehren des Grossen Krieges (chap. ii.). (L. J.) MINNESOTA (see 18.348). The pop. of the state in 1920 was 2,387,125 as against 2,075, 708 in I 9 I : an increase of 311,417, or 15 % for the decade, as against an increase of 3 24,3 14, or 18- 5 % for the preceding decade. The total white pop. was 2,368,936, of whom 1,882,772 were natives and 486,164 foreign-born. Negroes numbered 8,809 and Indians 8,761. The density was 29-5 per sq. m.; 25-7 in 1910. The urban pop. (in places having over 2,500 inhabitants) was 1,051,593, or 44-1%, in 1920, 41-0% in 1910, and the rural pop. 1,335,532, or 55-9%; 59-0% in 1910. The following table gives the pop. and the percentage of increase of cities having more than 10,000 inhabitants in 1920.

Cities

1920

1910

Increase per cent

Minneapolis St. Paul. . . . Duluth. . . . . Winona. . . . St. Cloud. . . Ribbing. .

380,582 234,698 98,917 I9-H3 15,873 15,089

301,408 214,744 78,466

18,583 10,600 8,832

26-3

9-3 26-1

3-o 49-7 70-8

Rochester. . . Mankato. . Faribault. . . Austin ... .

13,722 12,469 1 1 ,089 10,118

7,844 10,365 9,001 6,960

74-9 20-3 23-2

45'4

Agriculture and Minerals. There were 178,478 farms in Min- nesota in 1920, an increase of 14-3% since 1910. These farms cov- ered about 30,000,000 ac., two-thirds of which was improved; the total value of farm land and buildings was $3,787,420,118. The state's two most valuable crops in 1919 were Indian corn, of which 84,786,096 bus. were raised, the value being 110,221,931; and wheat, 37,616,384 bus., the value being $88,398,508; the total value of the principal farm crops was $506,020,233. Live stock on farms was valued at $293,373,818. The northern part of the state is developing rapidly as a stock-raising and dairying section. About 60% of the output of iron ore in the United States is mined in the three great iron ranges of northern Minnesota. The Mesaba range, no m. in length, embraces 180 active mines. The Vermilion and Cuyuna ranges combined form an additional iron belt of about 50 miles. A total of 43,263,240 tons of ore, valued at $144,706,532, was shipped from Minnesota mines in 1918.

Manufactures. In 1914 Minnesota ranked thirteenth among the states of the Union in the value of its manufactures. In the 5,974 establishments (not including hand industries) then existing, $354,434, '77 was invested, 115,690 persons were employed, and products valued at $493,354,136 were manufactured. The value of the products had increased 20 % since 1909. The state's fiye most important industries, in the order of the value of products in 1914, were the flour-mill and grist-mill, slaughtering and meat-packing, lumber and timber, dairy and creamery, and foundry and machine-shop industries. In 1919 some 300 flour-mills produced 29,337,131 bar. of flour; 8n creameries 143,176,204 Ib. of butter; and 85 cheese factories 9,452,191 Ib. of cheese. In flour and butter Minnesota's products exceed those of any other state. The total value of all dairy products in 1919 was $155,438,698. Since 1910 the slaughtering and meat-packing industries, centred at South St. Paul, have shown rapid growth. Because of the depletion of her forests Minnesota dropped from third place among the states in lumber manufacture