PRACTICAL DIRECTIONS FOR THE CON

STRUCTION OF BAROMETERS.

IT appears from many experiments, that the quicksilver stands higher in tubes of larger than in those of smaller bore; and, therefore, when observations are made with different barometers, some regard should be paid to the difference of their diameters; and it would be desirable to have them constructed of tubes of the same in terior dimensions. The tubes should be pretty large, in order to prevent the effects of cohesion or capillary attraction: they ought, for example, to be not less than one-fourth of an inch in diameter, and if they are one-third of an inch, they would be the better. Moreover, if a cistern be used instead of the leathern bag, its surface ought to be larger in comparison with the area of the section of the tube, in order that the addition or substraction of the quicksilver contained between the greatest and the least he ghts may not sensibly affect its depth. The tube should be preserved free from dust till it is used, and for this purpose it may be hermetically sealed at both ends, and one end may be opened with a file when it is to be filled If this precaution has not been observed the inside should be well cleaned, by washing it with spirits of wine, highly rectified, and by rubbing it with a little piston of shamoy leather, fastened to a wire.

The quicksilver should also be very pure; it may be purged of its air by previously boiling it in an earthen pipkin, closely covered. When the tube has been uniformly heated throughout, and rendered electrical by rubbing, the hot quicksilver should be poured into it, in a regular current, through a glass funnel, with a long capillary tube, so that the air may not have room to pass between the parts of the quicksilver. De Luc reccommends that the quicksilver should be boiled in the tube, as the most effectual method of purifying it of its air

and moisture.

The process is briefly this:-choose a tube of about a quarter of an ich in diameter, and not exceeding half ine in thickness; fill this within two

a

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inches of the top with quicksilver, and hold it with its sealed end over a chafing dish of burning charcoal, presenting first the sealed end to the fire, and then moving the whole obliquely over the dish As the quicksilver heated, the air bubbles appear like so many studs on the inner surface of the tube, which gradually run one into another, and ascend towards the higher parts of it which are not heated. Here they are condensed, and almost disappear, but after successive displacements, they at length accumulate in one place, and acquire a bulk by their union, which enables them to escape.

In performing this operation, it will be found, when the quicksilver boils, that the parts strike against each other, and against the sides of the tube with such violence, that a person unaccustomed to the practice is ready to apprehend that their force is sufficient to break the tube, of which, however, there is no danger. The quicksilver is thus freed from all heterogenous particles contained in it, together with their surrounding atmospheres; and the air which lines the inside of the tube, which cannot be expelled in any other way, is thus discharged. When this last mentioned stratum of air is thus expelled, the tube may afterwards be emptied and filled with cold quicksilver, and it will be found nearly as free from air as before.

pared will, by a certain quantity of The quicksilver in tubes thus preheat rise higher than in those made in the common way; and different instruments thus made will agree better with each other. In common barometers, in which this process has not been observed, there will frequently be found a difference of several lines, at the same time, and in the same situation.

De Luc found that barometers made in the way we have described rose uniformly in a heated room; while the quicksilver in those that had been prepared in the common way deWhen the room was suffered to cool, scended, and in different proportions. the former descended uniformly as they had risen while the latter ascend

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ed irregularly as before; and frequently, at the end of the experiment, they did not agree with each other as they did before it. The reason is ob vious, it proceeds from the effects of heat on the air, remaining in unequal quantities in the tubes in the one case, and on the purer quicksilver in the other. Another circumstance also requiring to be attended to in the con struction of these instruments, is the temperature of the air; for unless this remains the same the immersion of a given quantity of quicksilver will be variable, and its attitude an uncertain measure of the barometrical pressure. Lond. Mec. Jour.

PERPETUAL MOTION CLOCKS.

We never like to detract from the merit of a man who introduces to the public improvements on any subject of science or mechanism, but it becomes a duty to interfere when we see a plagiarism palmed upon the world as an original invention For the last few years there has been exhibited in the window of a watch-maker near the Royal Exchange, a clock which is wound up and continued in motion upon the balance system, by balls running backwards and forwards, and truly we believe a patent has been taken out for this very new principle What will our readers say when we inform them, that in a book upon me chanism, published at Rome in the year 1651, there is this invention, with a plate describing as accurate ly as possible, the clock which an Englishman has introduced in the nineteenth century as o: iginal.

Lon. Mec. Reg

PLATE POWDER.

In most of the articles sold as plate powder, there is an injurious mixture of quicksilver, which is said in some instances to so far penetrate and render silver bittle, that it will even break with a fall. Whiting, proper ly purified from sand, applied wet, and till dry, is one of the easiest, safest, and certainly the cheapest of all plate-powder. Jewellers, for

small articles, seldom use any thing else If, however, the plate be boiled a little in water, with an ounce of calcined hartshorn to about three pints of water, then drained over the vessel in which it was boiled, and afterwards dried by the fire, while some soft linnen rags are boiled in the liquid till they have wholly imbibed it, these rags will not only when dry, assist to clean the plate, (which must afterwards be rubbed bright with leather) but also serve admirably for cleansing brass locks, finger-plates, &c. ib.

AN UNALTERABLE WHITE FOR WATER

PAINTING.

Dissolve four unces of Roman alum in as small a quantity of hot water as is barely sufficient, and then mix it with two ounces, or two ounces and a half of honey. Set this mixture to evaporate to dryness in an earthen vessel, over a gentle fire. It will then appear like a spongy sort of coal, which being removed from the fire, must be pounded, and the powder placed in shallow crucibles or cuppels, so that it may lie very thinly on them. Expose these to a strong red heat for an hour. After this the powder must be pounded again, and being replaced in the cuppels, it must be exposed anew to a strong red heat, and to a free current of air for an hour longer. Being then removed from the fire, it is reduced upon a porphyr to an exceedingly fine powIt may der of an intense whiteness. be mixed with gum water, in the same manner as other paints are usually treated; and is not apt, like whitelead, to turn to a dusty hue. WM GATWARD. Lond. Mec. Mag.

HOTTENTOT MODE OF TANNING.

HOTTENTOT near the Cape form their tanning vats of the skin of some quadruped. The one which Mr Burchell observed was the skin of a zebra supported by four stakes on a frame, to which ts edges were bound by thongs, in such a manner that the middle, hanging down, formed a ca

The sheep leather, thus tanned. is made use of in the distant and more unfrequented part of the colony. for various parts of clothing, by the Hot tentots and the poorer class of colonists; but for the making of trowsers it is every where in general dem nd. Men's Jackets, and even women's gowns and petticoats, are made of it by those who are unable to buy woollen and linen clothing Such dresses were, before the English had posses sion of the land, the common costume of the peasantry and Hottentots, but since that time the prosperity and riches of the country have increased so much, that by far the greater part of the colonists can now afford to wear a dress entirely of English manufac ture.

Lond. Mec. Jour.

WATCH KEYS.

SIR,-The change in the shape of Watch Keys was not occasioned for mere fashion alone, but for the convenience of wearing. The old shape was so crooked as to be inconviently liable to catch ladies' gowns in dan cing, or when at any time assisting them; and by the entanglement, in very many mechanical operations, it caused the watch to be jerked entire ly out of the fob, more particularly before braces or suspenders were used, which was about the same time that the form of the watch keys was changed. In riding, especially in leaning forward to open a gate, the key, from its many angular shapes (somewhat as crooked as the Isle of Man arms,) often got fixed under the pommel of the saddle and on rising, the watch was drawn out suddenly, or the chain broke.

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The present form is doubtless the most convenient for wearing, though certainly not for winding; but as the wearing is for twelve hours or more, and the winding for less than half a minute custom has naturally, in my opinion, given rise to the present shape

vented to remedy the inconveniences A triple-jointed steel key was inof both the a ove-mentioned, which, when suspended and at liberty, was tion it was of the old shape. Gold or in a vertical posture but on applicacopper, however, is not strong enough for joints so small, and a steel key seals and chains, or even gilt ones, does not harmonize well with gold though this kind is now much in use with the lower classes in the country, particularly ploughmen and carters, mostly accompanying a steel chain.

AGRICOLA. Lond. Mec. Mag.

of the focus,

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to be ground together with the finest flour emery, until the surfaces exactly coincide. If the focal distance is very short, the plates, before they are soldered upon the lead, should be hammered as truly as they can be done, into the proper form.

If the lens is not for achromatic in struments, glass of a straw colour, whose dispersive power is as small as possible, is chosen, which has the two surfaces parallel: and, by means of scissors or pincers, it is cut into a circle, the edge smoothed by a common grindstone, and it is fixed by means of pitch to a wooden handle of less diameter than the glass, and about an inch high, so that the centre of the handle may exactly coincide with the centre of the glass. If the intended focal distance is very small, the surface of the glass is ground on the grindstone, so as to suit the gu ge as far as possible.

The glass being thus prepared, and supposing the lens intended to be convex, which is the most common form, it is then ground with fine emery upon the concave tool, which is to be firmly fixed to a table or bench, and the glass wrought upon it with circular strokes, so that its centre may never pass beyond the edges of the tool. After every circular turn, two or three cross turns along the diameter of the tool, in different directions, are given.

When the glass has got into its proper shape, and touches the tool in every part of its surface, which may be easily known by inspection, the emery is to be washed away, and

finer kinds substituted, until all the scratches and roughness are worn down. Those that remain after the finest emery has been used, are taken away, and even a slight polish given to the glass, by grinding it with pounded pumice stone. During all this operation of grinding, the convex tool, at the end of every five minutes, is ground for a few seconds upon the concave tool, in order to preserve the proper curvature. The glass is then separated from the handle by a knife, the pitch removed by rubbing it with a little oil, the already ground side fixed upon the handle, and the other side ground and finished in the same manner.

To form concave glasses, the convex tool is used in the same manner as the concave tool is used for convex glasses.

Some persons, for concave glasses, use leaden wheels, having the same radius as the curvature of the glass, and with their circumference of the same convexity as the glass is to be concave. These wheels being fixed upon a lathe the glass is held steadily in the hand, and ground upon them with emery. For common purposes, convex glasses are ground by fixing the concave tool upon the lathe, and applying in the same manner the glass to it. But this manner of grinding will not do for glasses when they are to be employed in the best kind of optical instruments.

The mode of polishing the glasses thus ground will be given in a succeeding Number.

Lond. Mec. Jour.