Sir Humphry Davy and Mr. Ure's proportions are different from all these. Probably some of your intelligent Correspondents, or the new Chemical Society, might throw some light on this obscure subject, and fix the determinate proportions to be used in procuring the chlorine gas-the theory of the chemical action that takes place during its disengagement, and the new combinations that are formed? The theory of chlorine is, most probably, still in its infancy.

MECHANICS' INSTITUTIONS.

We extract the following letter from the Plymouth and Devonport Weekly Journal, on account of some very sensible observations which it contains respecting the best course of instruction to be pursued in Mechanics' Institutions, and which we think well deserving the attention of all concerned in the promotion or management of these excellent establishments. The writer of it is, we understand, George Harvey, Esq. F.R.S., to whom Plymouth and the neighbouring town of Devonport are, in a great measure, indebted for the

....

..1 part....2 parts....3 parts.

establishment of their flourishing Mechanics' Institutions.

"

To the Editor of the Plymouth and Devonport Weekly Journal.

SIR, The letter contained in your Paper of Thursday last, on the subject of the future instruction of the Operatives is, perhaps, one of the most important that has yet appeared on the subject, since it goes at once to the root of the matter, by plainly telling us that the Members of the Mechanics' Institute, established in these towns, want not speculative theories and showy experiments in natural philosophy to be brought before them, but such wholesome practical information and occupations, and at the same time as will at once apply to their trades encourage habits of reflection and thought. Now and then, it is true, an occasional discourse on astronomy and chemistry may help to diversify, but the principal feature of every course of lectures should be practical information. This is what the operatives understand by a Mechanics' Institute, not only in this place, but in London, in Glasgow, and in every "other town in which the measure has been attempted; and I feel persuaded that the peculiar objects for which these fustitutions were designed, never will be accomplished unless the diffusion of solid practical information be made the basis of each course of instruction. Our workmen do not require to be

taught, at least for the present, the nature of chemical affinity, or the laws of electrical action. Lectures

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like these may amuse for a season, but, in the end, the operatives will ask, what have these things to do with constructive carpentry, with the diagonal braces of Sir Robert Seppings, or the forging of iron? We want, they will say, practical dissertations on the business and pursuits of active life, delivered in a plain and familiar way-in fact, in "our own way," in our common and familiar tongue; we want no hard names-no Latin or Greek-none of the pop-guns and toys of scienceno phantasmagoriæ or magic lanterns, but the honest, homebred, practical truths of knowledge, brought down to our apprehensions by the clearest, plainest, and most convincing means. A course of lectures of this kind, sprinkled here and there with a few of the leading truths of astronomy, chemistry, and of some of the more elementary doctrines of political economy, would meet precisely the views of the genuine founders of Mechanics' Institutions. On the latter science, in particular, both masters and workmen require to be much better informed than they are known to be at present, particularly on that part which relates to the wages of labour. At present the masters and journeymen are perpetually opposing each others interests. The journeymen combine against the masters, and the masters against the journeymen; and that man would be rendering a most important service to his country, who, in a short compass, and in a cheap and popular form,would unfold to the operative classes of the community some of the leading doctrines of wages and labour.

Your intelligent Correspondent has very properly adverted to the fact, that the greater part of the operatives of Plymouth are connected with housebuilding; and I will take upon me to say that, in few towns, is there so lamentable a deficiency, both in masters and workmen, as exists in this place, with respect to the most essential principles of the art. How few among our masons have any knowledge of the properties of an arch, or any notion of estimating the pressure and strain it is subject to! How few of our carpenters possess the requisite knowledge for fixing the dimensions of a beam of any kind, if the circumstances under which the timber is to be placed differ from the ordinary course of their experience! How often have I seen whole

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ranks of workmen stand amazed when they have been required to state the dimensions of a timber destined to bear a given load! I mention these circumstances, to inform those who have undertaken to be the Directors of Mechanics' Institutions, what is expected at their hands, what the operatives will demand from them, if their subscriptions are not so applied as to accomplish the great purposes for which those Institutions were designed.

Institutions for the instruction of Mechanics form the most remarkable class of societies ever proposed to the consideration of man; aud I do not remember if even Lord Bacon, in his magnificent description of Solomon's house (an essay which led to the formation of the Royal Society), at all attempts to provide for the instruction of the working classes, at least in the way the true and genuine supporters of Mechanics' Institutes now contem

plate. And of the multitude now so happily in favour of the diffusion of elementary knowledge among the operatives in different parts of the kingdom, how few have ever thought of the proper means by which these great ends are to be accomplished! Some men patronize Mechanics' Institutions for one reason, and some for anothersome for the love of a little fleeting popularity, others for the love of power -some that they may occupy the chief places in the synagogue, and be an object of amazement and wonder to the yet uneducated mechanic; but some, it is to be hoped, for the higher and nobler purpose of doing good, of benefiting their fellow-men by their exertions, and to prepare, for future generations, a rich harvest of happiness and joy. Of these several classes each will meet with its reward, for there is surely an all-seeing and ever: watchful eye keeuly surveying the troubled surface of the moral and intellectual waters, watching their rising volumes, and the operations of that swell which is slowly covering the waste of sands behind, and, at the same time, producing new currents in the deeper flood,

The author of the letter here adverted to has likewise alluded to the class of persons from whom the mechanics are likely to derive the most advantage; and here, too, I cordially agree with him in opinion, that a man familiar with their practical habits and thoughts is likely to prove the most useful lecturer. Men may, indeed, be profoundly versed in the mysteries of

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analysis, and the most complicated relations of ddefinite proportions, and make bat sorry instructors of a mechanics class, Men of transcendent at tainments can but rarely bend their exalted intellects to the wants and conceptions of

which pree unlearned. The truths

insuperable obstacles to lesser minds, are perceived with the rapidity of intuition by them. They bound from one fact to another with the rapidity of light, and in a few hours comprehend that which ordinary minds can never surmount. And thus it may happen, that a man whose mind is accustomed to reel, as it were, amidst the highest flights of philosophic fancy, may find the mighties of this conceptions fall lifeless at the feet of the mechanic. The difficulty, therefore, is to suit the lecture to the peculiar habits and wants of the persons the instruction is intended for; and my only object in offering these remarks, is to recommend to those who may hereafter become the instructors of the operatives, not only in this town, but wherever a Mechanics' Institution may be formed (and I trust every village in the kingdom will at least pos

sess,

in a short time, a Mechanics' Library), to adapt the lectures, as much as possible, to the habits and comprehension of those for whom they are intended-to couch them in language entirely plain and unadornedbut, above all, to select subjects in which the operatives may be likely to feel an interest and pleasure. This course I have shown is a difficult one; but it is an object which can be accomplished, if the guidance of those praiseworthy Institutions be undertaken by those who have their interest truly at heart, and not usurped by men of narrow and grovelling minds.

Finally, I would remark, in confirmation of what I have advanced, that it is to the strict observance of these principles that the Mechanics' Maga. zine owes its unparalleled circulation among the working classes. The papers contained in this most useful publication have, in general, been so truly practical and elementary, and the language in which they have been delivered so evident, plain, and familiar, that not only the workmen have been highly interested with its contents, but also many of the higher classes. But if the editor of this cheap publication bad consulted fame rather than utility, by seeking for dissertations and essays from the learned→→ had he invited Wollaston, end Hers

chel, and Buckland, to adorn its pages, the work would most unquestionably have failed in its object, even had it been rendered at its present convenient price. There is a language and a mode of thought peculiar both to the learned and the unlearned. The terms that were necessary to Newton, in the pursuit of his profound specu lations, were for a time inexplicable even to those who had advanced something beyond the borders of philosophical inquiry. Nor is any fault to be attributed to the philosopher on this account, since the obscurity in which new discoveries seem for a time to be involved, is a necessary concomitant of human improvement; and hence it is, that while science is perpetually enlarging its boundaries, it at the same time tends to simplicity and concen tration; and that the investigations which crowned the highest efforts of Archimedes, can now be compre. hended by fresh men at College and hence also it is that the economists have indulged in their magnificent speculations respecting the ultimate destiny of man-speculations so heartcheering and delightful, and so conducive to the comfort of those who entertain them, that even, although they were founded in delusion, 66 a wise man," says Dugald Stewart, “would be disposed to cherish them.”

COMMON SENSE.

Plymouth, June 27th, 1825.

DEFECTS OF OPTICAL INSTRUMENTS.

SIR,-Some time ago there appeared in the Mechanics' Magazine (p. 101), an inquiry for a method of removing the prismatic colours from the appearance of the planet Venus through a telescope.

Dr. Smith, in his Optics, voleb, p. 145, recommends to darken the eye-glass with the smoke of a candle; and in vol. 1, p. 417, he mentions, that when Venus was very near the sun, the splendour of the atmosphere took off so much, from the excessive lustre of Venus as to render her appearance better defined and freer from colours than' at any other time and place; and that the eye-glass of the telescope" need not then be smoked bFewod persons would be willing to smoke the eye glass of a valuable tele

scope, but the same purpose may perhaps be answered by interposing a piece of clear coloured glass, such as the screens of quadrants and sextants are made of. Some telescopes are unfit for astronomical purposes from the imperfections of the object-glass. Veins or streaks in the object-glass may be thus discovered Let the glass (detached from the telescope) be fixed or held up at about the height of the eye, at such a distance as to cause distant objects to appear inverted; then keeping the eye fixed upon some point of one of the objects seen through the glass, advance very gradually towards the glass, until that object appears confused, and spread all over the surface of the glass. If the head be then gently inclined to either side, every streak or vein in the glass will be very apparent.

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