was placed a 1 inch diameter wrought iron tube, with two slotted openings opposite to each other at its bottom end. Much care was taken to secure that the joints of these tubes, and especially of the 4 inch tubes, were perfectly watertight (this is most important).

The sheet iron lining of the boreholes was then withdrawn (this also is very important).

The tops of the 14 inch diameter tubes were all connected (Figs. 3 and 4, Plate XLI.) with the delivery end of a duplex pump, which forced down them the cooling fluid (hereinafter called the brine), which in French is called "l'eau salée," and is a 20 per cent. solution of chloride of calcium in water, unfreezable, except at an extremely low temperature. The tops of the 44 inch tubes were connected with the cistern in which the brine is cooled, and from which it is pumped by the duplex pump. The brine is cooled in the cistern, pumped down the 14 inch tubes. returns up the 44 inch tubes to the cistern, is again cooled there, and so on.

The brine is cooled by means of ammonia, which first passes through coils of wrought iron tube in tanks through which tanks a stream of cold water flows. The pressure of the ammonia in these coils is nine atmospheres above the atmospheric pressure, and it is then a liquid. It passes from these coils through a tube to coils of wrought iron tube immersed in the brine in the cistern abovementioned, in which cistern the brine is cooled. The ammonia expands in these coils in the cooling cistern to a pressure of three-fourths of an atmosphere above the atmospheric pressure, when it is an intensely cold gas which cools down the brine in the cistern—the temperature of the ammonia while passing through the coils in the cistern, is 22 degs. below zero Centigrade—the brine is agitated in the cistern by mechanical agitators to bring it to a uniform temperature.

From the coils in the cistern the ammonia is pumped by a gas-compressing engine, similar in principle to an ordinary air-compressing engine, to the coils in the tanks from which it started.

It will be seen that both the ammonia and the brine are continually circulating, and that there is no loss or waste of either.

The brine leaves the cooling cistern at a temperature of 12 degs. below zero Centigrade. It returns to it at a temperature of 9 degs. below zero Centigrade.

It should be understood that the production of the cold brine is outside of the Poetsch process, and of the Poetsch patent. The cold brine can be produced by sulphurous acid machines, or by carbonic acid machines instead of ammonia machines.

When the choice of a freezing machine has to be made for freezing a shaft, the preference should be given to a machine capable of producing a brine as cold as 20 degs. below zero Centigrade. This very low temperature will be very useful at the end of the operation, when the cold has to pass through very thick cylinders

of ice.

The working cost during the process of freezing is very small, the only items

being fuel for the boilers which supply steam to the gas-compressing engine and brine pump, and the labour of stoking at the boilers and attending to the engines.

On May 29th, 1891, freezing had been going on for 50 days, and the cylinders of ice in the shaft, and frozen ground outside the shaft, surrounding each of the 4 inch tubes were each 32 inches in diameter at the top, where only they could be examined.

It was supposed that they were larger in diameter at the bottom, because, of course, the brine was colder in the tubes at the bottom than at the top. It was expected that it would take about 70 days more before the cylinders of ice and frozen ground surrounding the tubes would have joined one another and formed a solid cylindrical mass of frozen ground and ice, containing all the 28 tubes.

Then sinking operations would commence, the eight tubes inside the area of the shaft being disconnected from the freezing apparatus and taken out as the sinking progressed, but the freezing operation being continued in the 20 tubes outside the area of the shaft so as to maintain a wall of frozen ground around the shaft until the permanent tubbing had been fixed.

Any leakage in the 4 inch tubes would be most prejudicial to the success of the freezing operation, as if the brine got mixed with the water in the ground it would render that water very difficult to freeze.

BRISTÖL IMPROVED PORTABLE ELECTRIC SAFETY-LAMP.

The improvements recently introduced by the patentee, Mr. L. Bristöl, of Bromley, Kent, into his portable safety electric lamp, a sectional view of which is illustrated in the annexed woodcut, chiefly consist in making

the casing of sheet steel, brass, or suitable metal, flanged around the top for the simple fixing by screws of the cover. This has an opening surrounded by a vertical wall, into which the vent-hole tubes of the secondary battery are projected and fitted with the Bristöl special non-splashing gas plugs, and the whole is protected by a swivelling cover plate, by which ready inspection and re-filling are attained.

Further, to prevent injury to the battery by accidental shocks, a space is left between it and the casing, into which are fitted wooden plates. On the front side the plate is grooved for the reception of

the wires from the battery to the glow lamp and the switch. The metal casing itself serves as the negative pole, and the on and off switch is arranged to

CUVELIER LOCK FOR SAFETY-LAMPS.*

The Cuvelier safety lock is intended to absolutely prevent miners from opening the lamps entrusted to them. The working of a bolt lock is controlled by the displacement of the ends of a flattened tube, similar to the tube of pressure gauge, which tube is arranged so as to receive liquid under pressure from an accumulator.

PITKIN PORTABLE BATTERIES AND HAND AND MINERS' ELECTRIC LAMPS.

The elements in the Pitkin secondary batteries are an improved form of the pasted lead plate. The active material is spread uniformly over both sides of the elements and being effectually held on by the peculiar form of the cast lead surfaces, the danger arising from the warping of the plates or the disintegration of the active material is reduced to a minimum. The elements are held the requisite distance apart by means of vulcanite pins screwed firmly into the positive plates. The whole of the connexions are protected from the corrosive action of the solution used by means of a thoroughly acid-proof cement.

Each cell is separate and self-contained. When the cells are joined up to form a battery they are placed within a polished oak or walnut wood case. The charging terminals are placed on the side opposite the lantern. A variable resistance is usually placed in the lamp circuit, and is used to regulate the flow of the current. The discharging terminals are either ordinary clamp screws or conical plug holes. The elements in the Pitkin primary batteries are of the simple zinc-carbon type

*

Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. xxxvi., pages 51-54, Plates VII. and VIII.