SPIIEIIOORAPII, an instrument invented iu 1856, for facilitating the practical use of spherics in navigation, &c., being a contrivance for constructing, without dividers or scales, any possible spheric triangle, and reading off the measures of the parts required; thus in nios cases saving much time and labour. The degree of aecuracy of the instrument ie limited only by its size, but it has been found by MA gators that circles of 5-inch radius will work any question which arises at sea, sufficiently near for the practical purposes of the navigator.
The description of the instrument will be better understood by giving seine general preparatory hints as to the names of the ordinary lines of the sphere.
In the following figure, No. 1, the observer is supposed to be at thei centre c of a hollow transparent sphere, on which the usual lines are drawn as upon a terrestrial globe. Ile would see the sun as at o upon the globe's surface amongst these lines. Rejecting all superfluous circles, ke., and confining our description to such parts as affect the sun's position at the time, we have in fig. 1 what is called a pro jection of the sphere on the plane of the meridian, the primitive circle representing the meridian of the place of the observer, u being the south part of the horizon a 11, and R being the north, z will be the zenith, N the nadir, c will be the east or west point of the horizon, r the north pole, a the south pole of the world ; a 1 parallel of altitude in which the sun is at the moment of observation, do the sun's decli nation, t ie a parallel of 18° distance below the horizon, limiting twilight to the period at which the sun is traversing from s where he sets, to c where he will be at midnight, d being his place at noon ; u d being the meridian altitude, P R. being the latitude (represented by the height of the pole above the horizon), then r z will be the co-latitude, o n the sun's altitude as measured upon the azimuth circle zn; oz will be the zeuith distance; of being the sun's declination as measured upon the hour circle Pf s, o r will be the polar distance, E Q the equator, z s the amplitude of the sun at setting—the angle R. ra being the time
of sun-set, the angle z P 0 the time of observation. The small circle d a is the semi-diurnal are, or half the length of the day, and a c the semi-nocturnal arc, or half the length of the night (c falling within n ec there will be no real night, only twilight), r s is the six o'clock hour circle, Z N the prime vertical, &c. In the spherograph only five terms are principally used, namely, latitude, declination, time, azimuth, and altitude, and what precedes will have fully prepared the mind of the reader for the application of the instrument to practical purposes.
The instrument is composed of two pieces of stout card-board, nicely attached, and revolving concentrically upou a pin carefully turned to work without lateral motion iu an ivory collar. The upper card has the ruled part, on which the lines are described, formed of stout trans parent tracing paper, and it is ruled like fig. 2; baying only azimuths and parallels of altitude (it has an oval space cut so as to enable the observer to put pencil marks on the under sphere), while the uncle one (fig. 3) has hour circles and parallels of declination : for of the three parts always given in every spheric triangle, two will fall on either one of the cards, and their intersection is made to touch the third datum as found in the other. We will suppose, in illustration, that we have given the sun's altitude 48°, the declination 10° N., and the apparent time 11 h. sm., to find the latitude. If we discard all lion ; and it has introduced a new mode of navigating ships in places subject to fogs and haze : for instance, the Montreal traders ass it on the Banks of Newfoundland, by substituting azimuth for altitude, in the three thiugs given, when the horizon is entirely invisible, the sun being in sight. It also dispenses with double alti tudes, inasmuch as latitude can be as well determined by it from a single observation as from two, rendering all elapsed time uncalled for. But it is not our purpose to describe these methods in detail. It seems, however, that a means of so readily finding the a fast-sailing steamer when approaching the land is important.