AND SURGICAL INSTRUMENTS.
North, North Central, and South Centra:, Galleries.
425
Kingdom.]
Brooke’s Self-registering Thermometer .
^hen 'l'' S a PP arat us is necessarily placed in the open air,12 'a a< ? tu al operation, it is provided with—to i nn er cylindrical zinc case, with sliding doors,
4er i ° tect the sensitive paper from light, when the cylin-hhie ^ eino ' p ed from, and brought back to, the photogra-
tvate' 4 n ou ter wind-and-water-tight zinc case, withd e ,. s r " tl ght doors, for removing and replacing the cylin-> and for trimming the lamps, if lamps are used.tr ai n 4 timepiece, to show the arrangement of thehand . 01 '^ er to avoid the unsteadiness of the hour-Play ’ f w ’hiich in ordinary movements results from theWhiop t ' le motion-wheels under the dial, the central axis"’hich Carr 3 es hour-hand is in the train, and the axisAs a. carr ies the minute-hand is placed out of the centre.a xis a 6 i° r ked or carrying arm is firmly attached to thetravel ri °L |er moveable hand or pointer is added, whichcomp* the former, and points to the hour. The
pos e d "^ting-bars of the balance of this piece are com-infin e ot brass and palladium, to prevent the rate being°f the?, by proximity to the magnets. The numbersof the f eaves . m the pinions are all prime to the numberseet h in the wheels with which they are in gear, tothe the chance of irregular motion from wear, as15 . °t the piece must necessarily be exposed.
°n a bra 11 e * ast ’ c sca le of vulcanized India-rubber, stretchedtime on +k I' ram e, for readily marking the subdivisions of16. g * . re g‘sters, which differ slightly in length.a Pparatns° 1Inens op * ; ^ le re gi s t el 's obtained by similar
the i ns 4 lithographic fac-simile of one day’s work of allthe V()1 “ m «™s employed at the Royal Observatory, fromObsevvaf 6 °* " Greenwich Magnetical and Meteorologicalf°r furthe° U ? Po - r to which the reader is referred
ltlv entov * m- ads ’ as well as to a series of papers by thePhical T ’ pubU! * ed in recent volumes of the “ Philoso-*■his app ai ?' 1 J sac !'i ons -” The most recent improvement ofada Pted to * S aI i aut °matic temperature compensation,the horizontal force magnetometer, 2; and to
the vertical force magnetometer, G. In the former instru-ment, this object is attained by approximating the lowerends of the bifilar suspension, by the excess of the ex-pansion of a zinc tube, over that of a glass rod : in theiatter, by the weight of a small quantity of mercuryenclosed in a thermometer tube attached to the magnets,passing from one side of the centre to the other.
[The skilful application of photography, by Mr. Brooke,to register natural phenomena, with no more labour thanthat of supplying the cylinder punctually with preparedpaper, is one of the most useful and beautiful uses to whichphotography has yet been applied. The paper is preparedso as to render it extremely sensitive to light, being firstwashed with a solution of isinglass, bromide of potassium,and iodide of potassium, in the proportion of 1, 3, and 2,respectively; and when required for use, it is washed withan aqueous solution of nitrate of silver, which causes thepaper to be sufficiently sensitive to the action of light, sothat if a beam of light be allowed to fall upon it, an im-pression is made upon that part where the light falls,which becomes visible on being washed with a solution ofgallic acid, with a small admixture of acetic acid. Alight is placed near a small aperture, through which rayspass and fall upon a concave mirror carried by a part ofthe suspension apparatus of the magnet, and this reflec-tion falls upon a piano-cylindrical lens of glass placed atthe distance of its focal length from the paper on thecylinder. As the magnet is ever varying, and makingsmall excursions on one or other side of its mean position,the point of light traces a corresponding zigzag line onthe paper. The thermometer apparatus has no mirrorand no reflector, the mercury in the tubes themselvesintercepting the pencils of light; and thus this apparatus,throughout the day and night, is constantly recording theslightest change of position of the magnets, and thesmallest changes of temperature.
The object of the self-registering magnetometer abovedescribed is to determine the direction and intensity ofthe earth’s magnetism. Its direction is generally foundby suspending a piece of steel previously magnetised, orin other words, a magnet, by parallel threads of untwistedsilk, and the bar settles in that position in which mag-netism causes it to rest, and which is called the magneticmeridian. The angle between the astronomical meridianand the magnetic meridian gives the magnetic declination,which is the subject of research with the declination mag-netometer ; at present this value in London is about 22 ^°west of the astronomical meridian.
Having determined the declination, the vertical planeis determined in which the force of magnetism is exerted.
The angle which the magnet makes, when freely sus-pended on this plane from the horizon, is termed the dip.At present, the dip at London is about 68° 40'. The forceof magnetism exerted in this inclined direction can be re-solved into two forces, the one acting in a horizontal direc-tion, the other in a vertical direction, so that conjointlythey shall produce exactly the same force as the singleforce. The bifilar, or horizontal force magnetometer, isintended for measuring the variations of the horizontalcomponent of the variations of the force of magnetism.It consists of a magnet suspended by two halves of a skeinof untwisted silk, kept at a certain distance apart. If anunmagnetized bar were thus suspended, it would remainat rest only in that position in which the two parts of thesuspension skein were without twist, and if it were turnedout of this position, it would endeavour to resume itsformer position, with a force proportionate to its weight,and the angle through which it had been turned. Thisprinciple is made the means of measuring the force of