476*

Class 10.—PHILOSOPHICAL, MUSICAL, IiOROLOGICAL,North, North Central, and South Central Galleries.

j-U nited

micrometer screw is, therefore, equal to one second ofarc. The positions of the azimuthal micrometers areunalterable; those of the altitude circle are fixed to theextremities of an arm which is moveable upon a centre,an arrangement which admits of their positions beingaltered relatively to the zero of the circle, and hence thereadings obtained upon particular objects are changed inevery new series of observations. Such errors as are dueto the graduation are by these means much diminished ifnot entirely eliminated. The axes are adjusted by meansof spirit-levels, in addition to which the instrument isfurnished with a plumb-line apparatus.

A portable altitude and azimuth instrument: each circleis 15 inches in diameter, graduated upon bands of silverto five minutes of arc, upon Mr. Simms’ new self-actingdividing engine. There are two micrometer microscopesto each circle showing single seconds of arc. The micro-scopes have achromatic object-glasses. This instrumentis furnished with the nadir point apparatus, or centralcollimator, the invention of the exhibitor, which consistsof a telescope in the interior azimuthal axis, around whichthe superior parts of the instrument revolve. The spiderlines, in the form of an acute cross upon the diaphragmof this central collimator, being placed in the principalforms of its object-glass, can be seen in the telescope ofthe instrument when it is directed downwards, or towardsthe nadir; and their intersecting point serves as an objectto which every observation made with the instrumentcan be referred, and therefore supersedes the use of anartificial horizon, or other extraneous means having thesame object in view. The transit collimation can also beadjusted by its means, without reference to any externalobject, and it supplies the place of the riding level for thetransit axis, in the event of its being broken.

A transit circle, two feet diameter, with two readingmicrometer microscopes having achromatic object-glasses ;the divisions upon the circle are subdivided to singleseconds by the micrometers. This circle was also gra-duated upon the self-acting dividing engine invented bythe exhibitor, as were also the circles of the altazimuthand the transit circle lately made for the Royal Observa-tory at Greenwich .

The telescope is achromatic, and has an aperture of3j inches, and a focal length of about 46 inches. Theapparatus by which the field of view is illuminated is theinvention of the exhibitor, and has been adapted to thegreat transit circle at the Royal Observatory. The fieldof view and the wires interchange the conditions of lightand darkness; hence, if the object be a bright one, thefield of view can be illuminated, and the wires appear asdark lines upon it; but if the object be a faint one, thefield can be made dark, and the wires luminous. Thischange is brought about instantaneously by either draw-ing outward or pressing, inward a small cylindrical plugplaced conveniently for the observer.

The illumination of the divisions upon the circle iseffected by the same lamp which illuminates the field ofthe telescope, as follows:—

A prism adjoining the microscope object-glass receivesthe light from a lens in the side of the lantern, which iscondensed upon the graduated face of the circle as a discof light, which just covers the extent of the field of themicrometer microscope, and is so arranged that a normalto the face of the circle bisects the angle formed by theincident and reflected rays.

The instrument is supported upon wooden models ofthe stone piers upon which it will be ultimately fixed;and the whole may be taken as a representative of theclass of instrument which is now rising in the estimationof British astronomers.

A transit instrument, 3£ feet focal length and 2jinchesaperture, with two setting circles upon the telescopetube, axis level, micrometer in the eye-piece, &c., as isusual in the most perfect instruments of this class; butas this instrument has been made for the coast survey ofthe United States , now being carried on under the direc-tion of Professor A. D. Bache, and is intended for obser-vations both in the meridian and prime vertical, theexhibitor has introduced two additional parts to fit it the

better for its work. Firstly, a reversing frame by whir ^the telescope can be lifted from the Y’s, turned end f°end, and again lowered into its bearings without beingsubjected to any handling, or other operation by whicthe parts may be unequally expanded and the adjus 'ments deranged; the operation is safe and expeditin’ 1 ?’Secondly, both the pivots are perforated, in one of whicha diaphragm with cross lines and an eye-piece is fittedand in the other an object-glass of suitable focal length ifixed, and hence the axis becomes a telescope. Now, 1this telescope be directed to any object, such as the croslines of a collimator or any terrestrial mark that presenitself, or is set up at a considerable distance from theinstrument, and the axis be made to revolve, the form °the pivots may be thereby examined, or (which in theinstrument exhibited, is its primary purpose) this tele'scope may be used for turning the principal telescope 90 >that is to say, for changing its meridional to a priffl evertical position, or the reverse.

This instrument is supported upon a cast-iron stand.

[The transit instrument consists of a telescope placedin the meridian. It is fixed to two arms, the extremiti eSof which are turned into two equal cylindrical pivots,which turn in Y’s on the top of two piers, placed east anwest of the centre of the telescope. The centres of th epivots, or their axes, are in the same straight line, whichis called the axis of the instrument. In order to observe

the instant that a celestial object passes the meridian,there is placed in the telescope, at the focus of the objectglass, a system of fine cobweb wires perpendicular to thehorizon, and one placed horizontal. At the instant anobservation is made, the star’s image coincides with theintersection of these wires. An arrangement of this kindis absolutely necessary', as the field of view of the telescopeis not a mere point.

The transit instrument is used in connexion with anastronomical clock, adjusted to sidereal time. A clock soadjusted that it completes a circuit in a sidereal day>_ aindicates 0h. 0m. Os., when the first point of Aries is ® nthe meridian, and having its dial-plate divided 1D .^24 equal spaces; hence, when the transit instrumentadjusted, and the clock goes correctly, at the instantfirst point of Aries is on the meridian, the time shown jthe clock is Oh.; when this point is IS 3 or 30° west of tmeridian, the time shown by the clock is 1 h. or 2respectively, and so on.—J. G.] ^

A diagonal transit instrument, in which the nieansilluminating the field of view is new in this class olstrument, and is the invention of the exhibitor. -

The mode hitherto adopted has been the placingreflecting surface in front of the object-glass, and rece ^ing light upon this plate from a lamp or candle place j ga distant part of the observatory. The objections tomode are, the difficulty of throwing light upon theSector at all, under such circumstances; besides, for ev ^observation, a new arrangement of the light has tmade; and further, by the reflector being placed in *iof the object-glass, part of its light is cut off. „1,

In the instrument exhibited, the light passes tm #

the axis, and this is effected by placing a lens, of win D( jvery large segment, nearly an annulus, projectsthe edge of the diagonal reflector within the axis. ^light, passing through a small lens fixed in one ° eD tpivots in the usual way', diverges upon the open segof the interior lens, and is thereby converged to tposite pivot, where the diaphragm is placed. n om er

An isometrical elliptograph invented by the AstroricalRoyal, G. B. Airy , Esq., by which ellipses in lsoD1 ura cy''perspective can be drawn with great facility and accThe exhibitor is the manufacturer. , ; eS ty’ s

Three imperial standard yards, made for Her ’’“’1 | 0 fCommissioners for the restoration of the StanWeights and Measures, with the supports invenProfessor Miller and the Rev. R. Sheepshanks.