a curious phenomenon observed on the summit of the Broken, one of the Harz mountains in Hanover. We have the following account of it by M. Hane. "After having been here (says he) for the thirtieth time, and having procured information respecting the above-mentioned atmospheric phenomenon, I was at length, on the 23rd of May 1797, so fortunate as to have the pleasure of seeing it; and perhaps my description may afford satisfaction to others who visit the Broken through curiosity. The sun rose about four o'clock, and, the atmosphere being quite serene towards the east, his rays could pass without any obstruction over the Heinrichshöhe. In the south-west, however, towards Achtermannshöhe, a brisk west wind carried before it thin transparent vapours, which were not yet condensed into thick heavy clouds.

"About a quarter past four I went towards the inn, and looked round to see whether the atmosphere would permit me to have a free prospect to the south-west; when I observed, at a very great distance towards Achtermannshöhe, a human figure of monstrous size. A violent gust of wind having almost carried away my hat, I clapped my hand to it by moving my arm towards my head, and the colossal figure did the same.

"The pleasure which I felt on this discovery can hardly be described; for I had already walked many a weary step in the hopes of seeing this shadowy image, without being able to gratify my curiosity. I immediately made another movement by bending my body, and the colossal figure before me repeated it. I was desirous of doing the same thing once more—but my colossus had vanished. I remained in the same position, waiting to see whether it would return; and in a few minutes it again made its appearance on the Achtermannshöhe. I paid my respects to it a second time, and it did the same to me. I then called the landlord of the Broken; and having both taken the same position which I had taken alone, we looked towards the Achtermannshöhe, but saw nothing. We had not, however, stood long, when two such colossal figures were formed over the above eminence, which repeated our compliments by bending their bodies as we did; after which they vanished. We retained our position; kept our eyes fixed on the same spot, and in a little the two figures again stood before us, and were joined by a third. Every movement that we made by bending our bodies..." bodies these figures imitated—but with this difference, that the phenomenon was sometimes weak and faint, sometimes strong and well defined. Having thus had an opportunity of discovering the whole secret of this phenomenon, I can give the following information to such of my readers as may be desirous of seeing it themselves. When the rising sun, and according to analogy the case will be the same at the setting sun, throws his rays over the Broken upon the body of a man standing opposite to fine light clouds floating around or hovering past him, he needs only fix his eyes steadfastly upon them; and, in all probability, he will see the singular spectacle of his own shadow extending to the length of five or six hundred feet, at the distance of about two miles before him."

If our memory does not deceive us, there is in one of the volumes of the Münchberger Transactions an account of a similar phenomenon observed by Dr. Ferrier, on a hill somewhere in England.

SPECULUM for reflecting telescopes. Under this title (Excell.) we have given the composition of the mix metal of which it has been found by experience that the best speculums are made; we have likewise given, under the same title, some directions for casting speculums: but owing to a circumstance in which the public can take no interest, we neglected to give directions for grinding and polishing them, and omitted some other circumstances, which, though not so important as these, are certainly worthy of notice. These omissions it is the object of this article to supply.

When the metal is taken out of the flask (See no. 3. of the article referred to), which it should be as soon as it has become cold, and while it is yet red-hot, care must be taken to keep the face downwards to prevent it from sinking. Holding it in that position by the git, force out the sand from the hole in the middle of the mirror with a piece of wood or iron, and place the speculum in an iron pot, with a large quantity of hot ashes or small coals, so as to bury the speculum in them a sufficient depth. If the sand is not forced out of the hole in the manner above directed, the metal, by sinking as it cools, will embrace the sand in the middle of the speculum too tight, so as cause it to crack before it becomes entirely cold. And if the metal is not taken out of the sand, and put in a pot with hot ashes or coals to anneal it, the moisture from the sand will always break the metal. Let the speculum remain in the ashes till the whole is become quite cold. The git may be easily taken off by marking it round with a common fine half round file, and giving it then a gentle blow. The metal is then to be rough ground and figured.

It may be proper, however, before we proceed to describe that process, to give an account of another composition for the speculum of a reflecting telescope, which has been employed with great success, by Rochon director of the marine observatory at Brest. Of this composition the principal ingredient is platinum; which, in grains, must be purified in a strong fire by means of nitre and the salt of glass, or that flux which in the English glas-houses is called by the workmen fandifer. To the platinum, when purified, add the eighth part of the metal employed in the composition of common specula; for tin without red copper would not produce a good effect. This mixture is then to be exposed to the most violent heat, which must be still excited by the oxygen gas that disengages itself from nitre when Speculum thrown into the fire. One melting would be insufficient; five or six are requisite to bring the mixture to perfection. It is necessary that the metal should be in a state of complete fusion at the moment when it is poured into the mould. By this process I have been enabled (says our author) to construct a telescope with platinum, which magnifies the diameters of objects five hundred times, with a degree of clearness and distinctness requisite for the nicest observations. The large speculum of platinum weighs fourteen pounds; it is eight inches in diameter, and its focus is six feet. Though the high price of platinum will, in all probability, forever prevent it from coming into general use for the speculums of telescopes, we thought it proper to notice this discovery, and shall now proceed to the grinding of the speculum.

For the accomplishing of this object, a very complicated process is recommended in Smith's Optics, and one not much more simpler by Mr. Mudge in the 6th volume of the Philosophical Transactions; but according to Mr. Edwards, whose speculums are confessedly the best, neither of these is necessary. Besides a common grindstone, all the tools that he made use of are a rough grinder, which serves also as a polisher, and a bed of stones. When the speculum was cold, he ground its surface bright on a common grindstone, previously brought to the form of the gage; and then took it to the rough grinder.

This tool is composed of a mixture of lead and tin, or of pewter, and is made of an elliptical form, of such dimensions, that the shortest diameter of the ellipse is equal to the diameter of the mirror or speculum, and the longest diameter is to the shortest in the proportion of ten to nine. This rough grinder may be fixed upon a block of wood, in order to raise it higher from the bench; and as the metal is ground upon it with fine emery, Mr. Mudge, with whom, in this particular, Mr. Edwards agrees, directs a hole or pit to be made in the middle of it as a lodgement for the emery, and deep grooves to be cut out across its surface with a graver for the same purpose. By means of a handle, fixed on the back of the metal with soft cement, the speculum can be whirled round upon this grinder so rapidly, that a common labourer has been known to give a piece of metal, four inches in diameter, so good a face and figure as to fit it for the stones in the space of two hours. The emery, however fine, will break up the metal very much; but that is remedied by the subsequent process of honing and polishing.

When the metal is brought to a true figure, it must be taken to a convex tool, formed of some stones from a place called Edgedon in Shropshire, situated between Ludlow and Bishop's Castle. The common blue stones, used by many opticians for this purpose, will scarcely touch the metal of Mr. Edwards's speculums; but where they must be employed for want of the others, as little water should be used as possible when the metal is put upon them; because it is found by experience that they cut better when but barely wet, than when drenched with water. The stones, however, from Edgedon are greatly preferable; for they cut the metal more easily, and having a very fine grain, they bring it to a smooth face. These stones are directed by Mr. Mudge to be cemented in small pieces upon a thick round piece of marble, marble, or of metal made of tin and lead like the former composition, in such a manner, that the lines between the stones may run straight from one side to the other; so that placing the teeth of a very fine saw in each of these divisions, they may be cleared from one end to the other of the cement which rises between the stones. As soon as the stones are cemented down, this tool must be fixed in the lathe, and turned as exactly true to the gage as possible. It should be of a circular figure, and but very little larger than the metal intended to be figured upon it. If it be made considerably larger, it will grind the metal into a larger sphere and a bad figure; and if it be made exactly of the same size, it will work the metal indeed into a figure truly spherical, but will be apt to shorten its focus, unless the metal and tool be worked alternately upwards. On these accounts, Mr Edwards recommends it to be made about one twentieth part longer in diameter than the speculum, because he has found that it does not then alter its focus; and he earnestly dissuades the use of much water on the hone pavement at the time of using it, otherwise, he says, that the metal, in different parts of it will be of different degrees of brightness.

When the metal is brought to a very fine face and figure by the bed of stones, it is ready to receive a polish, which is given to it by the elliptical rough grinder covered with pitch. With respect to the consistence of this pitch, Mr Mudge and Mr Edwards give very different directions. Whilst the former says that it should be neither too hard nor too soft, the latter affirms that the harder the pitch is, the better figure it will give to the metal. Pitch may be easily made of a sufficient hardness by adding a proper quantity of rosin; and when it is hardened in this way, it is not so brittle as pitch alone, which is hardened by boiling. Mr Edwards advises to make the mixture just as hard as to receive, when cold, an impression from a moderate pressure of the nail of one's finger. When the elliptical tool is to be covered with this mixture, it must be made pretty warm, and in that state have the mixture poured upon it when beginning to cool in the crucible. Our author recommends this coating to be made everywhere of about the thickness of half-a-crown; and to give it the proper form, it must, when somewhat cool, be pressed upon the face of the mirror, which has first been dipped in cold water, or covered over with very fine writing paper. If it be not found to have taken the exact figure from the first pressure, the surface of the pitch must be gently warmed, and the operation repeated as before. All the superfluous pitch is now to be taken away from the edge of the polisher with a pen knife, and a hole to be made in the middle, accurately round, with a conical piece of wood. This hole should go quite through the tool, and should be made of the same size, or somewhat less than the hole in the middle of the speculum. Mr Edwards says, that he has always found that small mirrors, though without any hole in the middle, polish much better, and take a more correct figure, for the polisher's having a hole in the middle of it.

The polisher being thus formed, it must be very gently warmed at the fire, and divided into several squares by the edge of a knife. These, by receiving the small portion of metal that works off in polishing, will cause the figure of the speculum to be more correct than if no such squares had been made. Mr Mudge directs the polisher to be strewed over with very fine putty; but Mr Edwards prefers colcotar of vitriol. (See that article, Encyclo.) Putty (says he) gives metals a white lustre, or, as workmen call it, a silver hue; but good colcotar of vitriol will polish with a very fine and high black lustre, so as to give the metal finished with it the complection of polished steel. To know if the colcotar of vitriol is good, put some of it into your mouth, and if you find it dissolves away it is good; but if you find it hard, and crunch between your teeth, then it is bad, and not well burned. Good colcotar of vitriol is of a deep red, or of a deep purple colour, and is soft and oily when rubbed between the fingers; bad colcotar of vitriol is of a light red colour, and feels harsh and gritty. The colcotar of vitriol should be levigated between two surfaces of polished steel, and wrought with a little water; when it is worked dry, you may add a little more water, to carry it lower down to what degree you please. When the colcotar of vitriol has been wrought dry three or four times, it will acquire a black colour, and will be low enough, or sufficiently fine, to give an exquisite lustre. This levigated colcotar of vitriol must be put into a small phial, and kept with some water upon it. When it is to be used, every part of the pitch-polisher must be first brushed over with a fine camel's hair brush, which has been dipped in pure water, and rubbed gently over a piece of dry clean soap. The washed colcotar of vitriol is then to be put upon the polisher; and Mr Edwards directs a large quantity of it to be put on at once, so as to saturate the pitch, and form a fine coating. If a second or third application of this powder be found necessary, it must be used very sparingly, or the polish will be destroyed which has been already attained. When the metal is nearly polished, there will always appear some black mud upon its surface, as well as upon the tool. Part of this must be wiped away with some very soft wash leather; but if the whole of it be taken away, the polishing will not be so well completed.

With respect to the parabolic figure to be given to the mirror, Mr Edwards affirms us, that a very little experience in these matters will enable any one to give it with certainty, by polishing the speculum in the common manner, only with cross strokes in every direction, upon an elliptical tool of the proper dimensions.