and afterwards polished by means of putty or oxide of tin, spread on pitch, as a covering to the same tool in the place of the emery. To grind a Speculum of six or eight inches in diameter was a work of no ordinary labour; and such a one used to be considered of great size. Mr. Ramage's Reflecting Telescope-the mirror of which is fifteen inches in diameter, with a focal distance of twenty-five feet, was until lately, the largest in use. Sir J. Herschell's, which stands in the place of his father's large one, has a Speculum eighteen inches in diameter, and a focal distance of twenty feet. With the difficulties of the undertaking, and the small success of his predecessors, Lord Rosse was fully acquainted, but he set about the work with a zeal that was a presage of his triumph. It is perhaps unnecessary to state how fitted his Lordship is in every way to accomplish a work which requires the combination of so many qualities-talent to devise-patience to bear disappointmentperseverance-profound mathematical knowledge-mechanical skill-uninterupted leisure from other pursuits; and yet these would not have been sufficient if a great command of money had not been added. Fortunately the world has

seen them all combined, and their application to the highest branch of scientific enquiry. The fact of the great Telescope alone having cost certainly not less than twelve thousand pounds, shows how few individuals could have successfully brought so large an instrument to a happy conelusion; and if to this be added the money expended on the Telescope with the Speculum three feet in diameter, and the various sums laid out in experimenting, we will see what a splendid offering has been made at the shrine of science by one of her most devoted admirers.

After many trials as to what combination of metals was most useful for Specula, both as regarded whiteness, porosity and hardness, Lord Rosse found that copper and tin, united very nearly in their atomic proportions, viz:-copper, 126.4 parts to tin 58.9 parts, was the best. This compound, which is of admirable lustre and hardness, and has a specific gravity of 8.8, he has used with both the small and large Specula, and he finds it to preserve its lustre with more splendor, and to be more free from pores than any other with which he is acquainted. Having ascertained the proportion to be used, he set about the casting of the Speculum. The difficulties attending this process were such, that instead of having the Reflector, which was to be three feet in diameter, in one piece, he tried the expedient of casting it in sixteen separate portions; the mirror when entire, presenting the form and appearance of figure 1. When cast,

the pieces were fixed on a bed composed of zinc and copper, -mixed in the proportion 2.75 of the latter to 1 of the former-a species of brass, which expanded in the same degree by heat as the pieces of the Speculum themselves; they were then ground as one body to a true surface, and when polished were found to answer remarkably well. The particulars in the manufacture of this compound Speculum requiring attention are, First-The method employed to find the quantity of zinc and copper which would when mixed expand in the same degree as the Speculum itself. Secondly -The form into which the brass was made. Thirdly-The manner in which the Speculum was joined to the bed. Fourthly-The method in which the pieces were cast. And Fifthly-the grinding and polishing.

In order to find the proportion of the zinc and copper, a bar of the Speculum metal fifteen inches long and one and a quarter thick, was joined accurately to a bar of the brass of the same length, but only three-quarters of an inch thick; they were then placed in a vessel of water, whose temperature was lowered to 32° F. and a fine hair line drawn evenly and continuously across both; the temperature of the water was then raised to 212o F. and during the expansion, the line was examined with a microscope; when it remained perfectly strait across both Speculum metal and brass, it was evident that they had equally expanded, and that the desired compound had been attained. There was much difficulty experienced in the melting of the brass, as the zinc being volatile it was in less quantity after this operation than before it, and the loss it sustained was not

always the same. After many endeavours to remedy this defect, it was found that a certain result was obtained by spreading over the metal a layer of powdered charcoal two inches in thickness; the loss sustained was always in this case constant, and amounted to 10th of the zinc employed. The bed was cast in eight pieces, each being of the shape of figure 2, which is seen in reverse. The depth of the bed

was five inches and a-half, and it weighed four hundred and fifty pounds. The eight portions were soldered together along the sides, and iron bolts used to secure the joinings more perfectly. However, it was found when the Speculum was fastened on that the joints had given in different places,