We next find Bramah turning his attention to the invention of a lock that should surpass all others then known. The locks then in use were of a very imperfect character, easily picked by dexterous thieves, against whom they afforded little protection. Yet locks are a very ancient invention, though, as in many other cases, the art of making them seems in a great measure to have become lost, and accordingly had to be found out anew. Thus the tumbler lock--which consists in the use of moveable impediments acted on by the proper key only, as contradistinguished from the ordinary ward locks, where the impediments are fixed-- appears to have been well known to the ancient Egyptians, the representation of such a lock being found sculptured among the bas-reliefs which decorate the great temple at Karnak. This kind of lock was revived, or at least greatly improved, by a Mr. Barron in 1774, and it was shortly after this time that Bramah directed his attention to the subject. After much study and many experiments, he contrived a lock more simple, more serviceable, as well as more secure, than Barron's, as is proved by the fact that it has stood the test of nearly eighty years' experience,* [footnote... The lock invented by Bramah was patented in 1784. Mr. Bramah himself fully set forth the specific merits of the invention in his Dissertation on the Construction of Locks. In a second patent, taken out by him in 1798, he amended his first with the object of preventing the counterfeiting of keys, and suspending the office of the lock until the key was again in the possession of the owner. This he effected by enabling the owner so to alter the sliders as to render the lock inaccessible to such key if applied by any other person but himself, or until the sliders had been rearranged so as to admit of its proper action. We may mention in passing that the security of Bramah's locks depends on the doctrine of combinations, or multiplication of numbers into each other, which is known to increase in the most rapid proportion. Thus, a lock of five slides admits of 3,000 variations, while one of eight will have no less than 1,935,360 changes; in other words, that number of attempts at making a key, or at picking it, may be made before it can be opened. ...] and still holds its ground. For a long time, indeed, Bramah's lock was regarded as absolutely inviolable, and it remained unpicked for sixty-seven years until Hobbs the American mastered it in 1851. A notice had long been exhibited in Bramah's shop-window in Piccadilly, offering 200L. to any one who should succeed in picking the patent lock. Many tried, and all failed, until Hobbs succeeded, after sixteen days' manipulation of it with various elaborate instruments. But the difficulty with which the lock was picked showed that, for all ordinary purposes, it might be pronounced impregnable.