lies air to the upper cavity, while the upper board is constantly pressing uniformlythe air in it. _ Although this is a considerable improvement upon the single bellows, it does not completely ob. viate the irregularity of the blast. So long as the lower board is not in action, the pressure on the up per board being uniform, the blast is the same. Eve ry time, however, the bottom board rises to force the air into the upper cavity, an extra pressure is given to the air in the upper cavity, and a temporary puff is produced. In the application of bellows to the smith's forge, the continued blast was of less importance than in the blast-furnaces applied to the smelting or refining of ores. The single bel lows are at present almost exclusively employed by anchor-smiths and cutlers, while the blacksmith and most others use double bellows, which are doubtless better for all purposes.
In France and other parts of the Continent, bel lows have been formed entirely of wood, instead of the flexible aides of leather, which serves to increase and diminish the capacity. The wooden bellows con sist of two boxes, each open on one side, the one being just capable of containing the other ; the outer box being placed with the mouth upwards, the other is made to descend into it, with the mouth down wards ; the latter being capable of moving up and down, while the other remained fixea. In the bot tom fixed box is a valve like the common bellows, and a pipe on the same level, to let out the blast. The change of capacity, by the motion of this box, causes the blast, and with less waste of power than that occasioned by the bending of the leather in. the common bellows. This advantage is, however, probablymore than compensated by the loss of air, from the box not fitting on the sides. See a description of this and some other th under PNEUMATICS, in the Ency Britan The common smith's bellows have lately been con-i structed of a circular form. The boards of these bel lows are round, and the moveable boards parallel to the horizon and to each other. We have given a' view of this construction in Plate XXXV. figures 4. and 5. A is the blast-pipe, B the moveable lower board, C the fixed board, into which the pipe is in serted, and D the upper moveable board, on which is placed a weight, to regulate the strength of . the blast. Motion is giien to the lower board by the lever L, and the chain H working on the rol ler R.
The form of these bellows being cylindrical, the weight required to give a given pressure and strength of blast will be easily determined. If the diameter be 1 foot, the area will be 118.19 inches. The most
convenient and proper blast for smith's bellows is about I lb. upon the inch, or from that to lb. The upper board, in this case, would .require a weight of 56.5 to give a blast equal to half a pound •upon an inch. This pressure would give a velocity equal to about 207 feet a second. If the diameter of the nose-pipe be changed, the number or length of the strokes, or both, must be changed, in order that the pressure and the corresponding density of the blast may remain the same. If the number and length of the strokes were kept up, and the aperture dimi nished, at the same time that the capacity of the bellows will not admit of enlargement, the pressure and density of blast will be increased, although no additional weight is laid on. This frequently hap pens in the smith's bellows when he makes an in creased effort to blow after the upper cavity is full. It is much better, however, not to exert the bellows in this way when a stronger blast is required, but to produce the efthot by an additional weight. A very strong blast is found to be very injurious to the iron when welding heats are required, and still more so in working steel. It is much better that an increase of air, which is frequently ranted, should be fur nished by increasing the aperture, supposing the power to beat the time adequate to keep up the in creased supply. Bellows should therefore be so constructed that the pressure may be uniform, and not immediately under the control of the workman. When he wishes to quicken his heat, he should have the means of increasing the aperture by a cir cular plate turning on an axis at right angles, to the length of the pipe, as seen in fig. 9. When in the position oh, the whole area is filled ; when in that of cd, the air in its full quantity. The index being p at any intermediate points, ff, will let in any proportionate quantity required.
The aperture might be made to change, by, the in crease of power upon the machine, and thus made to regulate itself. Several simple oontrivances of this kind -may be applied by any one skilled in ma chinery.
These improvements would render the common leather bellows, of the form above given, very useful for smiths. The irregular blast occasioned by their present construction is found to be very injurious to the iron, both as to its quality and economy. This is abundantly sholn in the use of some blowing machines lately invented, which have the advantage of a uniform blest, and at the same time much softer, being produced by a small pressure.