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THEORY AND PRACTICE OF ENGINEERING.

Book II.

Comparisons of the Effects of Mechanical Agents -A horse raising coals, by means of a

wheel and axle, moving at the rate of 2 miles per hour, and working 9^ hours per day,raised 1000 pounds avoirdupois to the height of 13 feet per minute : an overshot water-wheel, 10 feet in diameter, and served by 230 ale gallons of water per minute, a steam-engine with an 8-inch cylinder, and an improved one 6*12 inches in diameter, will do thesame work as a horse ; upon which data the preceding table has been constructed.

Of Blowers and Ventilators _There are several kinds of blowers, differing from each

other principally in the flexibility or inflexibility of their sides: a sack of leather orother substance, which is flexible and impermeable to the air, being filled with this fluidand compressed, would experience a pressure from the interior to the exterior* and if a tubeopen at both ends were attached to it, the air would be compressed in the tube, and wouldrush out with a velocity due to its elastic force.

The regularity of the blast which maintains the combustion of furnaces is of the greatestimportance in the success of metallurgic operations, and to obtain this it is requisite thatthe air-reservoir should be of such dimensions that the air which leaves the tube at eachstroke of the blower should be compressed into a very small volume with regard to thecapacity of the reservoir. The blower used at the Carron furnace is built of stone andvaulted ; its internal capacity is 445 cube metres ; the air which rushes out at each strokeof the piston is to the atmospheric pressure only as the 84th part of this capacity.

Whatever be the form of a blowing machine, the first points to be considered are thepressure of the air in the reservoir, the volume which escapes through the pipe at eachstroke of the piston, and the number of strokes in a given time. The volume of the airwhich passes in a unit of time, 1 second for example, from the pipe into the furnace beingdetermined, and divided by the area of the orifice of the tube, will give the velocity of theblast at the orifice. The greatest pressure of the air in the reservoir rarely exceeds theweight of a column of water 3 metres high, and may be measured by a column ofmercury 13 or 14 times less, the 3 metres of water answering to 22 centimetres of mercury:if we multiply the number of cube metres of air in the reservoir, which escapes through thetube in a given time, an hour for example, by the height of the column of mercury, and bythe density, 13*6, of this liquid, the product of the three quantities will express in units,each of one cube metre of water raised 1 metre, the dynamic effect which the blowing machinewould produce in an hour. The moving power applied to the bellows will, at the sametime, be capable of a greater effect, since it will have to overcome the inertia and frictionof the movable parts of the blowing machine: if we measure the pressure of the air inthe reservoir by a column of water, the height of this column, and the volume of airissuing from the reservoir, will be the two factors of the product which measures thedynamic effect of the blower. The object of ventilators is to renew the air in a givenplace, or rather to produce an artificial wind, by transporting the air from one space toanother. These two spaces communicate by a cylindrical tube, the extremity of which isfixed in the centre of a drum, formed by two circular parallel plates, and united by supportsin the circumference of these plates. A fly with 6 or 8 vanes turns in the interior ofthe drum; the air being driven from this is replaced by that in the cylinder, whence itfollows that it must be elevated from one extremity of the cylinder to the other, whichcommunicates with the drum: by placing a ventilator in a chest closed on all sides, anddivided into two compartments, the air in the chest receives such a motion that itcontinually passes and repasses from one compartment to another; ifj in the same chest,such substances as sulphur, charcoal, and saltpetre are reduced to powder, the air willcarry with it the finer particles, and separate them from those not sufficiently pulverised.The quantity of air to be raised from one place to another being given, the relation of thedimensions of the parts composing the ventilator can be established, and the moving powerrequired estimated. Ventilators are used to renew the air in low close places, such as pits,mines, cisterns, holds of ships, also in powder-mills to dry the powder in winter by currentsof warm air ; it has also been proposed to employ them to assist the evaporation of thewater contained in various preparations made from sugar, as syrup, &c. In the south ofFrance , a single cube metre of air, placed in contact with water, will evaporate threegrammes: thus knowing the number of cube metres which a ventilator can put in contactwith water in a given time, the quantity vaporised in the same time can be estimated.In coal-mines a portion of the small coal reduced to powder, and of little value in commerce,is used to maintain fires in the upper part of vertical shafts, which communicate withthe galleries, where the warm air rises and is replaced by the lower air, which causesa current in the interior of the galleries, towards the top of the shafts: if in the interiorof the shafts a horizontal axle were placed, carrying a fly, it is evident that the current ofhot air would turn the axle : by a similar method, in some kitchens, the hot air whichrises in the chimney serves as a mover to a smoke-jack.

We have considered air as a mover, and as a body to be moved : there is another pointof view in which we must regard it, as a resisting medium. We know that a bodyprojected into the air soon loses its initial velocity: the air at rest, struck by a body, first is