apparatus consists of seventy-two vertical pipes 5 feet long and 7 inches in diameter, fitted to an iron frame on the top of the upcast shaft: into each is inserted a steam pipe having a nozzle

inch in diameter, supplied with steam at 38 lbs. pressure. This rough apparatus exhausts 16,000 cubic feet per minute.

In the working of pneumatic tubes, between telegraph offices in London, the steam jet has been tested against a first-rate steamengine;1 at 40 lbs. pressure the engine does most work, but at 70 lbs. pressure the steam jet is superior. The nozzle, in this instance, is constructed in the most approved manner, and the vacuum produced is equal to 23 inches of mercury; but Mr. Morrison does not know of any instance where, with a water gauge of a few inches, such good results have been obtained by a steam jet as by other means. The jet is probably ill suited for exhausting large quantities of air at low pressures.

The Fan.-The best means of ventilation for tunnels seems to be the fan. It is used in collieries and mines throughout the world, and is the only machine that has ever been applied to tunnel ventilation.

The fan erected at Lime Street tunnel 2 is 29 feet 4 inches in diameter, 7 feet 6 inches wide, and runs at forty-five revolutions per minute, is by no means of the most approved construction, but the circumstances of the case are peculiar; at times the tunnel ventilates itself through the fan, which is constructed to allow of this, the heat of the boiler-fires assisting the natural ventilation. When the fan is at work the vacuum in the tunnel near the bottom of the shaft is only equal to 0.14 inch of water, but near the fan it is equal to 0.54 inch. When the air leaves the fan the pressure is equivalent to 0.19 inch of water above the atmosphere, this being the pressure required to drive the air through the chimney into the open air. The fan therefore seems to exhaust 431,000 cubic feet per minute, against a pressure of 0:54+0·19=0·73 inch of water, which represents 50 HP. The actual indicated HP. of the engine is 134. When running at forty-four revolutions, but doing no work, the indicated HP. expended in the friction of the engine and machinery is 34. The effective duty of the fan is therefore only 37 per cent. of the gross

1 Vide Minutes of Proceedings Inst. C. E., vol. xxxiii. p. 16.

2 Vide Inst. Mech. Eng. Proceedings, 1871, p. 24.

power, or 50 per cent. of the net power. The effective duty of many other fans is, however, much higher.

The pneumatic tube from Euston to the General Post Office is worked by a fan in Holborn,1 3,080 yards from Euston. This fan is 22 feet in diameter, and is driven at the rate of one hundred and sixty revolutions per minute. The tube is tunnel-shaped,

The usual speed of the
As the tube has an area

4 feet wide and 4 feet 6 inches high. carriage is about fifteen miles an hour. of 16-3 square feet, the discharge is 20,000 cubic feet of air per minute. The fan is arranged to work either for exhausting the air or for compressing it. At the speed mentioned the water gauge is about 10 inches,2 and this shows a useful power of thirty-two horses.

The Metropolitan Railway Company once made use of this tube for the purpose of ventilating their tunnel.3 The tube crosses the tunnel between Gower Street and Portland Road; and valves were arranged so that, on each journey of the carriage from Euston to Holborn, as soon as the carriage passed the Metropolitan tunnel, the valves opened, and the air was drawn from the Metropolitan railway instead of from Euston. But the tube could only be used when the fan was exhausting, and when the carriage was between the Metropolitan railway and Holborn; that is about once an hour for five or six minutes.

Of late years a great many fans have been introduced. One called the Guibal fan seems to give satisfaction. The effective duty of some of them has been stated to be 83 per cent. of the actual power put into the fan shaft; 5 but generally it is not so high. In these fans the casing is concentric with the fan, and quite close to it, with only one opening, the size of which is regulated by a shutter. The chimney is funnel-shaped, to allow the velocity of air to be reduced before entering the atmosphere. The fact that so much of the circumference of the fan is useless for discharge would lead to the supposition that the quantity of air must be less than that discharged from other fans of equal size.

Engineering, 23rd Aug., 1872. Engineer, 10th Nov., 1865.

2 Vide Min. of Proceedings Inst. C. E., vol. xxxiii. p. 4.

3 Engineer and Engineering, 31st July, 1874.

4 Vide Inst. Mech. Eng. Proceedings, 1869, p. 78. North of England Institute of Mining Eng., Trans., vol. xiv., p. 73. Engineering, 9th April, 1875.

5 Vide Inst. Mech. Eng. Proceedings, 1869, p. 152.

6 Mr. Cowper seems to hold the same opinion. Vide Minutes of Proceedings Inst. C. E., vol. xxx., p. 258.

It seems, however, that the useful effect of these fans is high, and that, for any given discharge and water gauge, the Guibal fan will work with less coal than many others, and with as little as any.

There is a fan of this description working at Thirslington Colliery. It is 36 feet in diameter and 12 feet wide, and at eighty revolutions it will discharge 80,000 cubic feet per minute, under a water gauge of 6.2 inches. A somewhat smaller fan, at Gethin Colliery, discharges 153,600 cubic feet per minute, under a water gauge of 2.6 inches. It is stated by Mr. Wilkinson that he obtained 63,000 cubic feet of air per minute from a Guibal fan, with a similar amount of steam required for 40,000 cubic feet per minute with Struvé's ventilator.

On their first introduction fans were supposed to be applicable only to low water gauges; but it has been found that they will work economically up to high gauges for ventilating purposes. There are some at Grand Busson, near Mons, in Belgium, 30 feet diameter, working at one hundred revolutions per minute with 7 inches water gauge.

To produce a current of air of 10 miles an hour through a tunnel 20 miles long, a water gauge of less than 7 inches is required, or, including an allowance of 1 inch for friction in the air-passages, &c., 8 inches altogether; it is therefore evident that fans are suitable for the pressures required in all practical cases; and as they are better adapted for passing large quantities of air than any other ventilating apparatus, they should be employed in all cases where tunnels are to be ventilated artificially. The best fans appear to utilize more than 70 per cent. of the actual power applied to the fan shaft, or 15 to 20 per cent. less than the indicated power of the engine.

The Blackman Air Propeller.-The Blackman air propeller is a fan. One 4 feet in diameter driven at a speed of 330 revolutions a minute, with an expenditure of one horse power, discharges 15,000 feet per minute.

Engineers have lately preferred to deliver air by pumps rather than by fans, the amount of air being known in this system with certainty. In schools and colleges in Dundee for example ventilating pumps are driven very economically by water motors; the pumps are rectangular wooden boxes, stiffened by iron ribs, and

provided at top and bottom with inlet valves consisting of a number of short waterproof cloth flaps working against a vertical wooden grid. A wooden piston with a vertical travel is held in place and worked by wire ropes above and below, which lead over pulleys to the water motor; the piston is balanced by a counterweight on the descending branch of the upper rope. A piston 5 feet square, with a stroke of 5 feet, works at 20 strokes per minute and delivers 150,000 feet per hour. These pumps have been designed by Mr. Cunningham, and when a greater volume of air is required he uses revolving pumps of the Roots blower type. These latter pumps are usually driven by a gas engine. The inlets are vertical. The air thus supplied may be warmed before entering the rooms by steam coils.

Cost of Mechanical as compared with Natural Ventilation.Mechanical ventilation is the only kind of ventilation which can be relied upon for large public buildings such as churches, schools, assembly rooms, theatres, and the like. In the valuable report of Professor Carnelley for the use of the school board of Dundee (1889) he compares the cost and efficacy of the systems of natural and mechanical ventilation, his chief conclusions being as follows:

In schools badly ventilated on the natural system, the microorganisms increase up to a certain point with increase of wall and floor space, whereas in mechanically ventilated schools where the air is quickly renewed, the micro-organisms decrease with increase of cubic space. This is shown in the following table:

The cost per head to naturally ventilate a school built to accommodate 1,000 children, as well as to heat, may be put at 23d. per head; the mechanical system will cost 71d., the difference being thus 197. 15s. per annum. The figures refer to Dundee, hence the cost will be greater in towns where coal is dearer, less where coal is cheaper than in Dundee. But in any case 201. or 251. per year per 1,000 children extra should be considered well spent if it conduce to greater purity of air. Carnelley investigated several systems both of heating and ventilating schools, and recommends a fan or fans driven by a gas-engine; also that the fresh air should be blown in not sucked out; that incoming air should be filtered through coarse jute cloth placed diagonally across the large inletflue or across the air chamber; that there should be but one main inlet air shaft, freely open at the top, and not fitted with Louvre boards, and lastly that the fresh air inlet shafts in the various rooms ventilated on the mechanical principle should be much wider and shallower than is usually the case, so as to distribute the air in a thin stream.