Maudslay's love of accuracy also led him from an early period to study the subject of improved screw-cutting. The importance of this department of mechanism can scarcely be overrated, the solidity and permanency of most mechanical structures mainly depending on the employment of the screw, at the same time that the parts can be readily separated for renewal or repair. Any one can form an idea of the importance of the screw as an element in mechanical construction by examining say a steam-engine, and counting the number of screws employed in holding it together. Previous to the time at which the subject occupied the attention of our mechanic, the tools used for making screws were of the most rude and inexact kind. The screws were for the most part cut by hand: the small by filing, the larger by chipping and filing. In consequence of the great difficulty of making them, as few were used as possible; and cotters, cotterils, or forelocks, were employed instead. Screws, however, were to a certain extent indispensable; and each manufacturing establishment made them after their own fashion. There was an utter want of uniformity. No system was observed as to "pitch," i.e. the number of threads to the inch, nor was any rule followed as to the form of those threads. Every bolt and nut was sort of specialty in itself, and neither owed nor admitted of any community with its neighbours. To such an extent was this irregularity carried, that all bolts and their corresponding nuts had to be marked as belonging to each other; and any mixing of them together led to endless trouble, hopeless confusion, and enormous expense. Indeed none but those who lived in the comparatively early days of machine-manufacture can form an adequate idea of the annoyance occasioned by the want of system in this branch of detail, or duly appreciate the services rendered by Maudslay to mechanical engineering by the practical measures which he was among the first to introduce for its remedy. In his system of screw-cutting machinery, his taps and dies, and screw-tackle generally, he laid the foundations of all that has since been done in this essential branch of machine-construction, in which he was so ably followed up by several of the eminent mechanics brought up in his school, and more especially by Joseph Clement and Joseph Whitworth. One of his earliest self-acting screw lathes, moved by a guide-screw and wheels after the plan followed by the latter engineer, cut screws of large diameter and of any required pitch. As an illustration of its completeness and accuracy, we may mention that by its means a screw five feet in length, and two inches in diameter, was cut with fifty threads to the inch; the nut to fit on to it being twelve inches long, and containing six hundred threads. This screw was principally used for dividing scales for astronomical purposes; and by its means divisions were produced so minute that they could not be