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The attainment of even an approximation to mechanical accuracy is a matter of great difficulty ; perfect accuracy is unattainable. This is, however, trite and well understood by mechanics in general; the reasons are nof so well understood. Why is it not possible to make two things precisely alike ? In vain the painter essays to reproduce a picture, or the sculptor to remodel a statue. In vain the counterfeiter strives to engrave a bank-note plate which will exactly resemble the one attempts to imitate. He may, in some rare instances, succeed so well as to deceive all inferior eyes, but he himself can perceive defects, and these defects cause him many fears and anxieties that others will discern them. Go to any heap of newly.struck coins, you can find no two which exactly resemble each other. The joiner lays out his work with the utmost care, and works to line as nearly as possible only to find that when the parts come together a shaving must be taken off here or a'joint is open there; some imperfection mars his work let him do the best he can. Now there must be some fundamental reason for this. What is it ? We find upon close analysis two physiological causes at work to prevent regularity and uniformity in anything we do. One is imperfect sensation, the other imperfect command of muscles. It is only by cultivatingin the highest degree the senses, and disciplining the muscles to become as much as possible subordinate to the will, that the artisan becomes skillful. These things accomplished, the physical education of a workman is completed ; all other things requisite may be acquired without manual practice, but practice alcne can perfect sensation and give power to the will over muscular motion. It may be said that much of the imperfection of workmanship arises from imperfections in implements : but it is easy to trace these imperfections to defective sensation and . execution. -It has only been by a gradual division and reductiohof imperfections, that we have obtained more perfect tools than savages use. From the stone used to crack nuts to the steel hammer of the present day a great many slow steps have been taken. How wide the difference between the auger and drill of modern times and the stone drill of the ancient races of North America; yet this difference has been attained by slow progression. Even yet our most delicately constructed instruments are not quite perfect. The two senses most to be charged with imperfect workmanship are sight and touch, but sight betrays us far more than all the others put together. In astronomical observation the habitual error in recording the instant of an astronomical event is ascertained as nearly as possible, and the formula expressing it is called the personal equation. This is allowed for in reducing all observations, and will generally be foimd pretty nearly constant. It amounts in seme cases to one half a second. The British mint allows twelve grains to the troy pound for variation in weight in coining ; and this may be taken perhaps as the measure of the nearest approach°to mechanical accn- raey in coining. It is fifteen seventy-seconds of one per cent. But there are other causes which lead to imperfection in workmanship not yet named. The variable textures of the materials used and the different thermometric and hygromet- ric conditions both of materials and tools, all tend to defeat accuracy. There are scarcely any two days in the year when a boxwood rule is precisely of the same length, and the variations in metallic rules are even greater than in those of wood. In very accurate drawing the draftsman finds it necessary to make a scale on the samo paper as that upon which the drawing is made, that the hygrometric expansion and contraction of the paper may not mislead the workman. Surveyors find errors creeping into their measurements from the expansion oftheirchains.; and we might go on to show that no material or implement can be made entirely free from one or the other of these adverse influences ; while many are subject to both. By clearly recognizing these facts, and with a full knowledge of the nature of materials and hoW they are affected by heat and moisture, the mechanic may attain very much greater accuracy than would otherwise be possible, no matter how skilled may be his eye and hand ; and it has been by attending to these nice points in combination with skill in other particulars that the che/-d'IP!U'lYre« of handiwork have been achieved.
