by the straps DD of hoop-iron, and so arranged that they can he tightened up hy the screw E. To the ring there is attached at G a pin with pointed ends, from the centre of which hangs the rod II carry ing, by means of the plate at K, the weights. The blocks are made to exert such a pressure upon the wheel that the pin G, from which the weights are suspended, remains always in the position shown, its pointed ends coinciding with the top of gauges upon the fixed wooden beams beside the brake. This apparatus is made to a certain extent self-adjusting by means of two compensating levers. The upper end ot one of these, M, moves freely through an eye N some dis tance below the centre of the shaft ; the hoop-iron rings are attached to its lower end at and P, the latter further from N than the former. If N coincided with the centre of the shaft the lever would cause no impediment to the motion of the ring along with the drum ; as it is actually placed, however, if any motion occur the point must move through a relatively smaller distance than in the other case, because the ratio ^ is less than p^- Tlic consequence of this is, that if through any cause the drum carry the ring round with it through any small distance, tends to tighten the belt, and P to slacken it, but the relatively larger movement of the latter causes the final result to be a slackening, so that the weight drops back intents proper place. If, on the other hand, the ring begins to slide back on the drum, a similar, but reversed, action at OP increases the pressure on the blocks, and the drum again gripping them, brings the weight back into its original position. The dash-pot L contains a piston very nearly fitting it, below which is water or oil, its object being to prevent too sudden a fall of the weight. It is really a little brake in itself, in which the energy given out by the descend ing weight is expended in overco?ning the factional resistance encountered by the water in passing upwards through the narrow space round the piston.
In order that a machine of this kind may be run continuously at a high velocity it requires careful and abundant lubrication. If the surface be too small, water will have to be used, but with a well- designed and not over-worked brake, tallow is the best lubricant.
With regard to the proper proportioning of surface, numerous ex periments with brakes of different sizes run at different speeds seem to show that the surface required varies as the energy transmitted, and (approximately) inversely as the peripheral velocity of the drum. The conclusions drawn from them, put in the most general form, are (for a brake having a cast-iron drum and wooden blocks) as follows: Let E be the energy (in foot Ibs) to be absorbed per minute (that is, the work done per minute by the machine driving the brake), T the number of revolutions of the dium per minute, E. its radius (in feet), and a the area (square inches), and b the breadth (inches) of the drum ; then in order that the latter may not heat, a should E not have a smaller value than 286 ?JT, while it is frequently arid very p properly made as much as 357 - r -, . Expressing the same relation in E E other terms, we have b from 0038 r^r, to "0048 ? j77p , or from E E 024 y- to 03 ^n- , V being the peripheral velocity of the drum in feet per minute. If the work be expressed in horses power (P) the equation is very nearly equivalent to J ? from to .
A brake may have automatic apparatus attached to it for showing or registering its speed or performance.
BRAMAH, Joseph, a practical engineer and machinist, was born at Stainborough, in Yorkshire, on the 13th of April 1749. He exhibited at a very early age an unusual talent for the mechanical arts, and having been incapacitated, when he was about sixteen, by an accidental lameness in his ankle, for the pursuit of agricultural labour, he was apprenticed to a carpenter and joiner. When the term of his engagement had expired he obtained employment for some time in the workshop of a cabinetmaker in London, and soon after established himself as a principal in that business. His first patent for some improvements in the mechanism of water-closets was taken out in 1783. In the following year he took out a patent for the peculiar locks which have long been named after him. His fertile invention led him to devise new arrangements for pumps, fire-engines, steam-boilers, and paper machinery, for all of which he obtained patents. The invention which has proved of most practical service, the hydraulic press, was first brought forward in 1796. Its principle is that of the hydrostatic paradox, and it has been found of very great use in all operations requiring the application of immense mechanical force. In 1806 Bramah patented a very ingenious printing-machine, specially adapted for bank notes, which was adopted in the following year by the Bank of England. During the latter years of his life Bramah erected some large machines at the Thames bank for sawing stones and timber, began to devise some im provements in bridges and in locks for canals, and was at one time actually employed in the execution of some water-works belonging to the department of the civil en gineer, which he completed with ability and success. His great and various exertions appear in some measure to have exhausted the strength of his constitution ; and his last illness was immediately occasioned by a severe cold, taken in the prosecution of his experiments in Holt Forest on the tearing up of trees. He died in his sixty-sixth year, on the 9th December 1814. (See notice of his life and works by Dr Cullen Brown in New Monthly Magazine, 1815.)
BRAMANTE, or Bramante Lazzari, one of the most celebrated architects of Italy, famous also as a painter, was born at Casteldurante, in Urbino, in July 1444. He showed a great taste for drawing, and was at an early age placed under a painter of some distinction, Fra Barto- lommeo, called Fra Carnavale. But though he afterwards gained some fame as a painter, his attention was soon absorbed by the sister art, architecture. He appears to have studied under Scirro Scirri, an architect in his native place, and perhaps under other masters. He then set out from Urbino, and proceeded through several of the towns of Lombardy, executing works of various magnitudes, and examining patiently all remains of ancient art. At last he reached Milan, drawn thither by the fame of the great Duomo, and remained there for several years. Informa tion as to this part of his life is singularly scanty, but he seems to have left Milan for Rome about 1500. He painted some frescoes at Rome, and devoted himself to the study of the ancient buildings, both in the city and in all the district as far south as Naples. About this time the Cardinal Caraffa, hearing of his studies in architecture, commissioned him to rebuild the cloister of the Convent della Pace. The celerity and skill with which Bramante accomplished his task gained for him the good offices of the cardinal, who introduced him to Pope Alexander VI. He began to be consulted on nearly all the great archi tectural operations in Rome, and executed for the Pope the palace of the Cancelleria, or chancery, which was much admired. But under Alexander s successor, Julius II., Bramante s talents began to obtain an adequate sphere of exercise. His first large work was to unite the straggling buildings of the palace and the Belvedere. This he accom plished by means of two long galleries or corridors enclosing a court. The design was only in part completed before the death of Julius and of the architect. So impatient was the Pope and so eager was Bramante, that the founda tions were not sufficiently well attended to ; great part of it had therefore soon to be rebuilt, and the whole is now GO much altered that it is hardly possible to decipher the original design.
Bologna, among which is specially mentioned by older writers a round temple in the cloister of San Pietro-a- Montorio, Bramante was called upon by Pope Julius to take the first part in one of the greatest architectural enterprises ever attempted, the rebuilding of St Peter s. Bramante s designs were complete, and he pushed, on the work so fast, that before his death he had erected the four great piers and their arches, and completed the cornice and the vaulting in of this portion. He also vaulted in the
principal chapel. After his death in 1514 his design was
