Another evil of grinding is, that it takes from the mechanic all sense of responsibility, and all spirit of emulation, while it deludes him with the idea that the surface will be ultimately ground true. The natural consequence is, that he slurs it over, trusting to the effect of grinding, and well knowing that it will efface all evidence either of care or neglect on his part.

It thus appears that the practice of grinding has altogether impeded the progress of improvement. A true surface, instead of being in common use, is almost unknown.* Few mechanics have any distinct knowledge of the method to be pursued for obtaining it, nor do practical men sufficiently advert, either to the immense importance, or to the comparative facility of the acquisition.

Due latitude must be allowed to the expression "true surface." Absolute truth is confessedly unattainable. Moreover, it would be possible to aim at a degree of perfection beyond the necessity of the particular case, the difficulty of attaining which would more than counterbalance its advantage. But it is certain that the progress hitherto made falls far short of this practical limit, and that considerations of economy alone would carry improvement many

*This, it must be borne in mind, had reference to the state of things existing in 1840.

degrees higher. The want of it in various departments of the arts and manufactures is already sensible. The valves of steam engines, for example; the tables of printing presses,stereotype plates,-surface plates,--slides of all kinds, require a degree of truth much superior to that they generally possess. In these, and a multitude of other instances, the want of truth is attended with serious evils. In the case of the slide valves of steam engines, there is occasioned a great loss of steam power, and also an immense increase of wear and tear.* In stereotype printing, inaccuracy of the plates renders packing necessary to obtain a uniform impression. A vast amount of time and labour is thus sacrificed, and the end is, after all, but imperfectly attained.

The extensive class of machinery, denomi

* Mr. Dewrance, superintendent of the locomotive department of the Liverpool and Manchester Railway, in a letter to Mr. Whitworth, dated the 23rd of December, 1840, says,"In answer to yours of the 20th inst. respecting the difference of the slide valves got up with emery, and those that are scraped or got up according to your plan, the difference is as follows:-I have this day taken out a pair of valves got up with emery that have been in constant wear five months, and I find them grooved in the usual way. The deepest grooves are one-eighth of an inch deep, and the whole surface, which is eight inches broad, is one-sixteenth hollow, or out of truth. Those that were scraped are perfectly true, and likely to work five months longer."

nated engine tools, affords an important application of the subject. Here every consideration combines to afford accuracy. It is implied in the very name of the planing engine. The express purpose of that machine is to produce true surfaces, and it is itself constructed of slides, according to the truth of which will be that of the work performed. When it is considered that the lathe and the planing engine are used in the making of all other machines, and are continually re-producing surfaces similar to their own, it will manifestly appear of the first importance, that they should themselves be perfect models.* There is, perhaps, no description of machinery which would not afford an illustration of the importance belonging to truth of surface, and at the same time, of the present necessity for material improvement; nor is there any subject connected with machines, the bearings of which, on public interests, whether manufacturing or scientific, are more varied or more extensive.

The improvement so much to be desired, will speedily follow upon the discontinuance of grinding. Recourse must then be had to the natural process. The surface plate and the

It is plain that, in machines intended to be used in reproducing other machines, errors in surface are of the utmost consequence, for the original defects are propagated in an aggravated form.

scraping instrument will come into constant use, affording the certain and speedy means of attaining any degree of truth which may be required. A higher standard of excellence will be gradually established, the influence of which will be felt throughout all mechanical operations, while, to the mechanic himself, a new field will be open, in which he will find ample scope for the exercise of skill, both manual and mental.* The subject will be best illustrated by a description of the process.

There are two cases for consideration, in reference to the preparation of surfaces,-the one, where a true surface plate is already provided, as a model for the work in hand, and the other, where an original surface is to be prepared.

The former case is that which will generally occur in practice. The method to be pursued is simple, and requires care rather than skill. Colouring matter, such as red ochre and oil, is spread over the surface plate, as equally as possible; the work in hand, having been previously filed up to the straight edge, is then applied thereto, and moved slightly to fix the colour, which, adhering to the parts in contact, afterwards shows the prominences to be removed

*It is satisfactory to be able to state that the results here anticipated have been long ago realized

by the scraping instrument, and the operation is frequently repeated. As the work advances, a smaller quantity of colouring matter is used, till at last, a few particles spread out by the finger suffice for the purpose, forming a thin film over the brightness of the plate. A true surface is thus rendered a test of the greatest nicety, whereby the smallest error may be detected. At this stage of the process, the two surfaces must be well rubbed together, that a full impression may be made by the colour. The higher points on the rising surface become clouded over, while the other parts are left more or less in shade. The dappled appearance thus produced, shows to the eye of the mechanic, the precise condition of the new surface in every part, and enables him to proceed with confidence in bringing it to correspondence with the original. Before this can be accomplished, however, the scraping instrument must be employed, the file not having the precision or nicety requisite to finish the operation. Experience will be a sufficient guide when to exchange the one for the other. It will be found, that when the parts to be operated upon have become to any considerable extent subdivided, scraping is much the more expeditious method. The instruments should be made of the best steel, and carefully sharpened to a fine edge on a