CLASS X.]

PHOTOGRAPHIC APPARATUS FOR SELF-REGISTRATIOX, At.

2S1

nates, is traced upon the paper by the action of a spot of ilight proceeding from another gas-lamp placed near thecylinder, and passing through a slit fixed to the carrier ofthe cylinder.

To the horizontal force magnet there are attached ap-paratus for correction of temperature.

That for the horizontal force magnet, which is describedin the Illustrated Catalogue, page 411 (Second Part), isbased upon the following consideration:—

Let (b) be the lower interval between the two divisionsof the suspension skein, the equation of equilibriumbeing—

then the arrangement adopted is to make the variation ofb, by the effect of temperature, equal to the variation ofm, arising from the same cause. This is attempted to beeffected by clamping to the magnet a glass rod, the endsof which are inclosed in two zinc tubes, whose adjacentends are separated by the width of the clamp. The zinctubes and glass rod are clamped together at such a dis-tance from the centre of suspension, that the ratio of thedifference of linear expansion of the length of glass andzinc, intervening between the clamps to the interval (6),may be equal to the temperature coefficient.

The clamps are adjusted in order that, when their posi-tion has been approximately determined by calculation,the error of position may be corrected by experiment.

As the expansion of glass and zinc are both taken to beuniform, and are at any rate far beyond the limits ofatmospheric change, this method cannot serve to represent the coefficient of (<), and the residual error of thecorrected instrument must be experimentally determined.*

The arrangement for the correction of the verticalforce magnet is made by clamping a small thermometerto the magnet, parallel to its axis, and in the same planeas the knife-edges. The middle point between the freez-ing point of the thermometer and the centre of the ballis placed opposite the centre of the magnet.

Let » he the weight that at a distance q from the lineof suspension would be equivalent to the temperaturecorrection for 1° Fahrenheit, or 32 ; w the weight ofmercury contained in one degree of the scale of the ther-mometer ; p the distance of the freezing point from thecentre of the ball, and 2 t the length of one degree of thescale; also let c x + s x i be the temperature coefficient ofthe magnet; then if

qv = pin, and c r.

The statical moment of the mercury displaced fromthe ball at any temperature x° above 32will be equiva-lent to cx + cx , and will represent the temperature cor-rection.

We will now proceed to detail the process of preparin'’-the paper, bringing out and fixing the photographic im°pression.

The paper used should be of a strong even texture, andprepared with attention, to the exclusion of all foreignsubstances which might combine injuriously with thechemicals used.

The first preparation of the paper is with—

Isinglass 4 grains.

Distilled water 1 fluid ounce.

The water is boiled, and a portion then poured uponthe isinglass, which, when dissolved, is poured into theremaining water, and all is boiled together. To this solu-tion, filtered, is added—

12 grains of bromide of potassium, and8 grains of iodide of potassium.

* The correction for temperature has hitherto, at nearlyevery place, except at Greenwich, been applied to mag-netical observations, on the supposition that the decreaseof magnetical force was proportional to the increase ofheat; but this is found to be very far from being the case.The correction for temperature of the horizontal forcemagnet at Greenwich is represented by 0-00009050 (t - 32)t 0’000000620 (t — 32-; the second term here becomes vervlarge, when the. temperature departs much from 32-, and inMr. Brooke’s arrangement is not taken into account.

One side of the paper is then washed with this solution,and dried quickly before a fire. The paper thus preparedwill keep iu a dry place for two months.

When the paper is required for use, it must be washedwith the following solution, in a darkened room with ayellow light:—

Crystalline nitrate of silver 50 grains

Distilled water 1 fluid ounce.

To bring out the impression, the paper is washed withthe following solution: —

Saturated solution of gallic acid 1 ounce.

Acetic acid a few drops.

To fix the impression, first wash the paper well withwater, then with the following solution:—

Hyposulphate of soda 1 drachm.

Distilled water 5 ounces.

And, lastly, wash carefully with water, and leave thepaper to soak for a short time. The impression is thenfixed, and light may be admitted.

A Council Medal was awarded to Mr. Brooke for thisbeautiful application of photography to the registrationof natural phenomena.

Wilton (No. 402, pp. 452, 453) has exhibited threeFox’s dipping-needles, furnished with needles of 6-j inches,4 inches, and 2 inches in length respectively. Thosewith the longer needles are identical in all particularswith those which have been used successfully on board ofships, both for the observation of dip and relative mag-netic intensity.*

Parkes and Sox (No. 671, p. 467) exhibit a number ofpocket-compasses.

Greex (No. 446, p. 462) has exhibited several magneticsun-dials, with metal and agate caps, adapted to everydegree of latitude. They are mounted in boxes made ofvarious materials. Mr. Green has also exhibited variousother sun-dials, some of which are fixed in round maho-gany boxes with levels and adjusting screws, and othersfitted up for north and south latitudes; also several hori-zontal sun-dials, and ivory circular thermometers, w ithcompass or magnetic sun-dials attached to them, Mr.Green also exhibits pocket-compasses with metal andagate caps, and others also with floating cards, &c., vari-ously mounted in round boxes with unproved hinges.Several compasses are fitted up in variously constructedboxes of ivory, brass, German silver, &c., together withothers in the form of a watch, of various materials, somebeing made of leather, others in gilt and silver cases.

Mr. Green also has exhibited cone-compasses and brassgimbal-compasses, intended for use in small boats.

Teates (No. 332, p. 446) exhibits a prismatic compassadapted to measure both horizontal and vertical angles,with spiral level attached in such a way as to be appli-cable in both cases.

Electrical Instruments.

The electric instruments are few in number, and thereis not one adapted for the purpose of determining thequantity of atmospheric electricity for meteorological pur-poses. This is a matter to be regretted, as the presentstate of meteorology greatly needs a simple, inexpensiveinstrument, adapted for the observation of atmosphericelectricity, and one which would be uniform in its actionunder uniform circumstances.

One exhibitor, Westmoreland, (No. 444, p. 462) has,however, the merit of exhibiting a gutta-percha electricalmachine, which holds out the hope of obtaining electricityof tension on a large scale by the application of steam-power, and thus a motive force, which may serve for themovement of machines, or enable us to accomplish objectsat present quite unforeseen, but which the habitual useof an electric pow er commensurate with that of lightningmay bring into view.

Westmoreland (No. 444, p. 462) exhibits a gutta-percha electric machine. It consists of two rollers ofequal diameters, placed one above the other, over whicha baud of gutta-percha, 4 inches in width, is stretched;

* For description, see Illustrated Catalogue.