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17.280-11.9975.283 pounds of the total pressure lost by this change of temperature. As the temperature does not change so much as 40°, in a short time it will not have so marked an effect upon the ventilation of mines: still, an increase of power will be required during the summer months in order to keep the ventilation uniform during the year.

From the above, it may be seen that natural ventilation may be more during the winter than during the summer months. Where furnaces or steam-jets are employed to produce ventilation, the longer the upcast, the better; as the longer upright column of light air gives rise to a brisker ventilation. For this reason, furnaces should not be used in shallow pits.

When coal is worked at a dip, the effect of natural ventilation is very complicated; and in many instances there will be little benefit derived therefrom at any season of the year, owing to the tendency of the heated air to ascend against the down current. Natural ventilation is not a help to artificial ventilation, but often is of very great hinderance to a fan, on account of the vacillating atmospheric changes. On this account, therefore, the inlet and outlet should be as nearly as practicable on the same level. High winds, directed by hills, blowing against the exhaust duct of a fan, greatly impede its action.

13. In the year 1814 Mr. Buddle, an Englishman, read a paper before a society formed for preventing accidents in coal-mines, illustrating the various modes employed in the ventilation of collieries by plans and sections.

At that time the only light used in coal-mines was

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the candle made of sheep or ox tallow, the latter being considered the better.

When the air in the mine became mixed with inflammable gas, the mode of determining its existence and degree of inflammability was described by Mr. Buddle as follows:

"In the first place the candle, called by the colliers 'the low,' is trimmed; that is, the liquid fat is wiped off, the wick snuffed short, and carefully cleansed of red cinders, so that the flame may burn as purely as possible. The candle, being thus prepared, is holden between the fingers and thumb of the one hand; and the palm of the other hand is placed between the eye of the observer and the flame, so that nothing but the spire of the flame can be seen, as it gradually towers over the upper margin of the hand. The observation is generally commenced near the floor of the mine, and the light and hand are gently raised upwards till the true state of the circulating current is ascertained. The first indication of the presence of inflammable air is a slight tinge of blue, a bluish-gray color, shooting up from the top of the spire of the candle, and terminating in a fine extended point. This spire increases in size, and receives a deeper tinge of blue, as it rises, through an increased proportion of inflammable gas, till it reaches the firing-point. The experienced collier

knows all the gradations of shew (as it is called), and seldom fires the inflammable gas, except in cases of sudden discharge."

When the air was highly charged with inflammable gas, the steel mill was resorted to. It consisted of a steel wheel, to which was applied a piece of flint when it was turned rapidly, thus throwing off a continuous succession of sparks, the light of which was rather uncertain however, it was a substitute. But as it required one man to work the mill for every man cutting coal, mining became too expensive; and only those portions of the mine were worked where a sufficient current of air could be brought to bear upon the gas to dilute it sufficiently to allow of candles being used.

In 1814 Dr. Clanny produced a lamp by which a light could be used in an inflammable mixture of gas with impunity. The insulation of the flame was accomplished by means of water; and, although the first lamp which was produced, it was too complicated and cumbrous for general use.

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In 1815 at the same time, but in distant localities - Mr. George Stephenson and Sir Humphry Davy both produced lamps which insulated lights in inflammable mixtures of fire-damp without exploding the gas externally. These productions have been of the utmost importance in coal-mining, and consequently to the commercial interests of the country generally.

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Mr. Stephenson reasoned, that "if a lamp could be made to contain the burnt air above the flame, and to permit the fire-damp to come in below in a small quantity, to be burned as it came in, the burnt air would prevent the passing of the explosion upwards; and the velocity of the current of the air from below would also prevent it passing downwards." He accordingly constructed a lamp of tin, with a hole in the bottom to admit the air to the flame, and a top perforated with holes. By experiments with this lamp he discovered the true principles of the safety-lamp.

Sir H. Davy, at about the same time, communicated. with a friend that he had "discovered that explosive mixtures of mine-damp will not pass through small apertures or tubes, and that if a lamp or lantern be made air-tight on the sides, and furnished with apertures to admit the air, it will not communicate flame. to the outward atmosphere." He subsequently found that “iron-wire gauze, composed of wires from onefortieth to one-sixtieth of an inch in diameter, and containing twenty-eight wires, or seven hundred and eighty-four apertures to the inch, was safe under all circumstances."

The process by which Mr. Davy arrived at the above conclusion is given by himself in a small work “On the Safety-Lamp for Coal-Mines, with some Researches