Your Committee are quite satisfied that the plan proposed by Sir John Robison is eligible, and that, when suitable tools have been provided for creasing them into their proper form, and the workmen have had experience in using them, they will be able to produce half-round files very superior to those cut in the usual way for certain kinds of work. The smoothness of the cut will give the new files a decided preference.

Your Committee beg to remark, that the file cut on the concave side is the only one they have ever seen. As to making three-quarter round or even cylindrical files on the principle suggested by Sir John Robison, your Committee see no objection;-difficulties may occur to the makers in the first instance, but it is to be hoped these difficulties may

come by experience.

be over

On the whole, your Committee conceive that Sir John Robison's plan of cutting and forming half-round and cylindric files with continuous teeth, will be a decided improvement in a certain class of files, and that the improvements are deserving of the marked approbation of the Society. JAMES MILNE, Convener.

EDINBURGH, 30th January 1843.

Extract from Letter Messrs JOHNSON, CAMMELL, & Co., Sheffield, to the late Sir JOHN ROBISON, K.H., F.R.S.E., February 13, 1843.

The plan we have hitherto adopted in turning the half rounds, has been by top and bottom hand-swages of block-tin, that is, by placing the heated file on the bottom swage or groove, and turning it by hammering the top swage or mandrel into the bottom groove. This was the simplest method we could adopt to try the practicability of your communication ; but we find it is not sufficiently powerful to effect and ensure in all cases a regular curve, and which we suspect has, in a great measure, been the cause of our failing in the hardening.

We have now ordered tools, say swages, &c. of copper, to be applied under the screw-press, and by obtaining the curve the whole length of the file gradually, and by one pressure, we hope will obviate our previous difficulties; and not leave those cranks or weak places, caused by the hand-blow, and which in all cases shrunk or twisted in the hardening. We expect in a few days to send you a few specimens from this process. We have also considered your plan of turning by copper rolls, but fear, from the causes herein named, it will not be very practicable. In all cases of steel-rolling, the steel has a strong tendency to follow and curl up after the rolls; in ordinary cases of plain steel it can be easily remedied and straightened; in this case the file would curl up after the rolls on the convex side, and in adopting any plan to keep it straight from the rolls, it would be liable to open out or disarrange the curvature of the file and injure the teeth. Again, we suppose in rolling there would be a danger of flattening or injuring the tooth of the file, unless the metal forming the rolls was nearly as soft as the heated steel; and if of the

same temper, the rolls would be continually out of order and require renewing.

We make these few remarks, desirous of putting you in possession of our mode of proceeding, and the reasons; and which may probably elicit some improvement from your scientific experience.

We could easily by various means turn the heated steel into any form of curve, but in adopting the means, the object in view must be to preserve the proper sharpness of the tooth, which in this heated state by undue pressure or by hard metals is so liable to be injured; we trust, however, our next specimens will prove that these difficulties have been JOHNSON, CAMMELL, & Co.

overcome.

JAMES TOD, Esq., Secretary, Royal Scottish Society of Arts.

SHEFFIELD, November 7, 1843.

Sir,-We are favoured with your esteemed letters of the 2d and 4th instant, with their enclosures, and are obliged by your kind attention.

We now have pleasure to hand you (as requested) extract from our letter of February 13th last, to the late Sir John Robison, explaining the mode we then adopted in the manufacture of the half-round files from steel of parallel thickness, as suggested by him, viz., by means of a screw-press and swages of copper, and which we see is the plan named in the paper communicated to the Society.

From the specimens sent you last week, you will perceive we have deviated from the plan first suggested" of cutting them from blanks of steel prepared as for a thin equalling-file." We, however, did not give up this plan until, from practical experience, we found its working very uncertain and irregular, for the file being of uniform thickness, the edges presented an equal or greater degree of resistance to pressure, than the centre; and the top swage or boss coming in contact with the centre of the file previous to any other part, caused it to bend more freely than the edges, producing various degrees of curvature in the same file. Again, if our top boss was so shaped as to create an earlier and freer pressure on the edges, to ensure a more uniform curve, we then endangered the sharpness of the teeth on those parts of each side of the file, convex and concave, first receiving such undue pressure. These objections and difficulties are all overcome or lessened by our present mode of cutting and turning the files from steel with slightly tapered edges, on one side, thus, The flat surface is cut with a continuous tooth, and can be turned either convex or concave, and the tapered surface can either be cut in ridges or left safe, or uncut, which, from the following extract, you will perceive was Sir John's first idea. In Sir John Robison's first communication, November 10, 1842, after recounting the difficulty of obtaining a smooth half-round file or one of equal continuous tooth, he says,—

"I propose to overcome this difficulty, and to cut half-round files as truly smooth as flat ones are now struck, by making the blanks of rolled steel-plate; by striking them in the flat state, and by afterwards giving them the degree of curvature required by means of a screw-press, and

tin or copper swages; or else by passing them between grooved and furrowed rolls (of soft metal for the struck side)," evidently implying that one side would be unstruck or uncut. In fact, for some time at first we only attempted to cut one side of the file, and frequently now we are requested to leave sometimes the concave and at others the convex surface blank or uncut.

We trust these crude remarks may be of interest, as giving a reason for our discontinuing to cut and press these files from steel of parallel thickness, and if you choose to embody any part of them in your report, you are at perfect liberty. We have not the slightest objections to Sir John's paper being printed in your Transactions; and if any of our remarks are not perfectly clear, we shall be happy in giving further explanations.*

It must be highly satisfactory for the friends of the late Sir John Robison (and your Society in so freely noticing it), to learn that the invention is considered by eminent engineers and practical men, one of the first and best improvements in file-manufacturing of the day. Always at your command, we are most respectfully, Sir, your very obedient servants, JOHNSON, CAMMELL, & Co.

An Account of Experiments with Thermo-Hydro-Electrical Currents, with an Examination of the Metals exposed to Thermo-Electric Action. By M. R. ADIE of Liverpool. Concluded from p. 353 of preceding Volume (Vol. XXXV.) Communicated by the Author.

19. The arrangements described (17 and 18) appeared to me to be worthy of further attention, to ascertain the variations in the amount of chemical action performed by two opposing thermo-electrical currents, which are much influenced by climate. The experiments made with these currents, now about to be given, have been conducted in the midst of a large town, where there are many local disturbing causes to affect the results ; they should be received, then, more for shewing the general action of the currents under examination, than as correct results for this climate. The season through which they have extended has been remarkable for passing from one extreme to another; beginning with a very cloudy, bad spring, next an indifferent summer, and then a fine autumn, the autumn including six weeks, from the middle of August to the end of September, of the most powerful sunshine for the season of the year ever remembered.

In April last a thermo-electric battery, fig. 1, was placed, with the upper half exposed to sun and sky, and the lower half screened; the voltameter contained silver poles, and argento-cyanide of potassium in solu

* Messrs Johnson, Cammell, & Co., obtained the Society's Honorary Silver Medal for their specimens of Files cut on Sir John Robison's method, 13th November 1843.

tion; it was attached near to the upper joints of the battery, on the south face, and, like them, varied its temperature with the weather. The poles weighed 1 grain each on 21st April. On 21st August, after four months' exposure, the poles were reweighed, when the difference in weight was found to be 0.12 of a grain; the negative pole for the day current being the heavier, shewing that the silver has been deposited by solar radiation.

20. When the voltameter attached to the battery is maintained at a uniform temperature of about 90° by means of a sand-bath, the current generated chiefly by astral radiation, preponderated over the day current in April, when the sun was more than twelve hours above the horizon. To examine the influence of the fluctuations of the weather on this arrangement: A battery, fig. 1, was placed with the upper half exposed to sun and sky; two copper wires were soldered to its poles, then conducted inside the house, and permanently attached to a galvanometer fixed at the distance of 15 feet from the battery. With this instrument I was enabled to watch through the summer the changes in the force and direction of the currents. The summary of these observations is, 1st, For a clear sky, the indications are constant as to direction; from one to one and a half hours after sunrise, to within the same period of sunset, the exposed joints of the battery are heated, and for the remainder of the twenty-four hours, the same joints are cooled by radiation. The time stated betwixt the rising or the setting of the sun, and the change in the direction of the thermo-electrical currents, is given as the average interval; for there are occasions when the time intervening betwixt the change of the currents and the rising or setting of the sun is much longer or shorter. The most frequent cause of these changes in this interval is, when the general aspect of the sky is clear, with clouds on the horizon near where the sun rises or sets. 2d, In dry cloudy weather the action of astral or solar radiation is much checked; but this state of the sky rarely alters the direction of the current due to the period of night or day at which the observation is made. 3d, In wet unsettled weather: Rain falling for hours without intermission, nearly suspends the action of the battery either by night or day. But by day the light or intermitting summer showers produce by far the greatest variations in the force and direction of the currents generated by the weather. The showery weather, so common during our last spring and summer, by wetting the exposed and previously warmed portion of the battery, produced a thermo-electrical current corresponding to that derived from the radiation to a clear midnight sky, and often exceeding it in intensity.

21. The continuation through the summer and autumn of the experiments where the battery, fig. 1, was employed to deposit silver in a voltameter at a uniform temperature of 90°, enables me now to state the observed changes through the four seasons. From January to the end of April there was a constant deposit of silver by the current due to astral radiation; the rate of this deposit gradually decreasing as the

spring advanced. From May to the 21st June the two opposing currents were on the whole period so nearly counterbalanced, that I could discern no action; but at the beginning of May the night-current slightly preponderated. In July, August, and September, the day-current was slowly depositing silver. October there was no perceptible action. November the current due to astral radiation again begun to deposit silver, and this action may be expected to continue till next spring or

summer.

These results differ considerably from the experiment where the voltameter was exposed like the battery; they shew a much feebler action through June and July, when the sun was nearly two-thirds of the time above the horizon, than in the first experiment (19); in it the opposing currents developed by day showers, and by astral radiation, are nearly powerless, on account of the reduced temperature of the voltameter (11). In the other, where the decomposing cell is maintained at an equal and elevated temperature, the current caused by day showers, or by astral radiation, is as effective as the solar radiation current. To the showery unsettled weather of May, June, and July, I can alone attribute the minute action observed; for in September, when the weather had changed to be remarkably fine, the solar current became more effective in depositing silver than it had been in June.

22. The galvanometer permanently attached to a thermo-electric battery (20), was at mid-summer exposed for two or three hours every morning to the solar rays; this deflected the needles of the instrument as much as 20°, when the helix was not connected with the battery. M. Becquerel and others have already noticed similar deflections, in experiments of this kind, where they were found to be caused by small uprising currents of heated air; but, at the time, as I thought that a change in the magnetic intensity of the upper needle, from its elevated temperature, and also local currents in the copper coils of wire, might operate to produce a part of these deflections, I removed first the coils of wire, which made no change; next the diaphragm or compass card interposed betwixt the two needles, so that the rays of the sun now acted equally on both of them; this alteration increased the deflection, shewing streams of heated air rising from the upper needle to be the active force which produced the deflection of 20° in the complete instrument, when in the sun's rays.

23. While engaged with the last experiments, they appeared to me to point out where to look for the explanation of a curious instrument, Mr Watt's sun needle, which I had seen many years before, under my father's care, rotating by the action of the solar rays; and as I am not aware that the cause of this rotation has yet been shewn, I will briefly state the result of some experiments, which refer it to small uprising streams of air, generated in their highest energy on metallic surfaces.

Mr Watt's instrument consists of a light piece of wood, with agate centre, in form resembling the needle of a mariner's compass; on the