regions far beyond those which have hitherto been made. But such improvements can never be effected, unless by numerous experimental investigations, conducted by those whose minds are thoroughly imbued with the principles of chemical and optical science.*

* One of the latest improvements on Achromatic objectglasses was made by a foreigner of the name of Guinand, who was originally a cabinet-maker. After acquiring a knowledge of the principles of optics, and of the mode of constructing telescopes, he applied himself particularly to ascertain the proper composition of flint-glass for achromatic purposes; and, after spending twenty or thirty years in making experiments-casting one pot of glass after another, and meeting with frequent disappointments,-he at length succeeded in obtaining glass for achromatic telescopes, of larger dimensions and of a quality superior to what could formerly be procured. Of this glass was formed the largest triple achromatic telescope ever constructed, which was lately erected in the observatory of the university at Dorpat, under the direction of M. Fraunhofer. This glass is perfectly free from veins, and has a greater dispersive power than any obtained before. The diameter of this object-glass is almost ten inches, and its focal distance twenty-five feet. It has four eye-pieces, the lowest magnifying 175 times, and the highest 700 times. Mr. Tulley of Islington lately constructed, of similar materials, manufac、 tured by the same artist, a telescope whose object-glass is about seven inches diameter, and its focal length twelve feet, which is now in the possession of Dr. Pearson. The piece of flintglass of which the concave lens was formed, cost Mr. Tulley about thirty guineas. Unfortunately for science, the ingenious artist (Guinand) is now dead, and it is uncertain whether he has left any particular details of his process behind him. The possibility, however, of procuring glass for the construction of very large achromatic telescopes is now put beyond a doubt.

The unscientific reader may acquire a general idea of an achromatic object-glass from the following figure,-where A D represents a double unequally convex lens of crown-glass, C B a double concave of flint-glass, and E F another convex lens

With regard to the invention and improvement of the steam-engine-a story has been told "that an idle boy being employed to stop and open a valve, saw that he could save himself the trouble of attending and watching it, by fixing a plug upon a part of the machine which came to the place at the proper times, in consequence of the general movement." Whether or not this story has any foundation in

of crown-glass. These are placed together in the manner re. presented in the figure, and form what is called an achromatic

object-glass, the term achromatic signifying free of colour. Sometimes only two lenses, a convex of crown, and a concave of flint-glass are combined for the same purpose. In the case of a single convex glass, the image formed is blended with the prismatic colours which come to foci at different distances from the lens, and consequently produce a comparatively indistinct image, which will not admit of a high magnifying power. But the achromatic lens, forming an image without colour, will bear a larger aperture, and a higher magnifying power, than a common refractor of the same length. So great is the difference-that an achromatic telescope of Dollond, only three feet ten inches in length, was found to equal, and even excel the famous aerial telescope of Huygens of 123 feet focal length, and the gentlemen present at the trial agreed that "the dwarf was fairly a match for the giant." The principal obstacle to their construction on a large scale, is, thè difficulty of procuring large pieces of flint-glass free of veins, and of a proper dispersive quality.

truth certain it is, that all the most useful improvements in this engine have been the result of the most elaborate researches and investigations of scientific truths. The first distinct notion of the structure and operation of this powerful machine appears to have been given by the marquis of Worcester, in 1663, in his Century of Inventions." Its subsequent improvements by Savary, Blackey, Newcomen, Beighton and Fitzgerald, were the results of physical knowledge, of mechanical skill, and of the most laborious investigations. Its latest and most important improvements by Mr. James Watt, were owing no less to the scientific knowledge which adorned his mind, than to his mechanical ingenuity. He was a man of a truly philosophical mind, eminently conversant in all branches of natural knowledge, and the pupil and intimate friend of Dr. Black, and had attended the lectures of that distinguished philosopher in the university of Glasgow. And he often acknowledged "that his first ideas on this subject were acquired by his attendance on Dr. Black's chemical lectures, and from the consideration of his theory of latent heat, and the expansibility of steam." We We may therefore rest assured, that all the future improvements and new applications of this noble invention will be the result of physical and chemical knowledge combined with mechanical skill; and consequently, no artizan can ever expect to be instrumental in bringing the steam-engine to its highest pitch of improvement, and in directing its energies to all the purposes to which they may be applied, unless the pursuits of science occupy a considerable share of his attention.

The first hint of the Mariner's compass is generally supposed to have been owing to chance. Some persons may have accidentally observed, that when

a small loadstone is suspended in water on a piece of wood or cork, its ends pointed towards the south and north. Such experiments seem to have been applied at first for mere amusement, and to excite astonishment in the minds of the ignorant and illiterate. But it was not till some genius possessed of science and of reflecting powers seized the hint thus given, that it was applied to the important purpose of directing the mariner in his course through the pathless ocean. And to science we are indebted for the manner of determining the declination of the needle, in all parts of the world, by means of the azimuth compass, and thus rendering it an accurate guide to the navigator in every region through which he moves. The discovery of that peculiar principle termed Galvanism, was partly owing to accident. Whilst Galvani, professor of Anatomy at Bologna, was one day employed in dissecting a frog, in a room where some of his friends were amusing themselves with electrical experiments, one of them having happened to draw a spark from the conductor, at the same time that the professor touched one of the nerves of the animal, its whole body was instantly shaken by a violent convulsion. Having afterwards suspended some frogs from the iron pallisades which surrounded his garden, by means of metallic hooks fixed in the spines of their backs, he observed that their muscles contracted frequently and involuntarily, as if from a shock of electricity. Such facts, presented to the view of unscientific persons, might have produced nothing more than a gaze of wonder; perhaps supernatural powers might have been resorted to in order to account for the phenomena, and in a short time they might have been forgotten as a vision of the night. But such scientific minds as those of Valli,

Volta, Monro, Fowler, Davy, Humboldt and Wollaston, having seized upon these facts, having contemplated them in every point of view, and instituted experiments of every description in relation to them -most astonishing discoveries in science have been brought to light the whole aspect of chemistry has been changed, and numerous improvements introduced into the practice of the useful arts.

Alkalis have been decomposed, new metallic substances discovered, the cause of the corrosion of metals ascertained, and the means determined by which it may be effectually prevented.

It is a truth which the whole history of science fully corroborates, that very few important discoveries have been made by accident or by ignorant persons, whose minds were not directed to the particular object of research. On the other hand, we have every reason to believe, that there are many facts and circumstances which have passed under the inspection of untutored minds, which, had they come within the range of men of science, would have led to many useful inventions which are yet hid in the womb of futurity, and which will reward the industry of more enlightened generations. The inventions to which we have now adverted, and many others, where chance suggested the first rude hints, would, in all probability, have lain for ages in obscurity, without any real utility to mankind, had not the genius of science seized upon them, viewed them in all their bearings, and traced them to all their legitimate consequences and results. Had the telescope, the steam engine, and the mariner's compass, in their first embryo state, remained solely in the hands of ignorant empirics, they might have been reserved merely as play-things for the purpose of vulgar amusement, or exhibited by