With respect to the train for giving the proper retro grade motion 16 the moon's nodes, and for showing her latitude at all times, this is also derived from the lunation in the following manner: The wheel 74 is made fast, under wheel 64, on the same fixed arbor of the cross bridge,and . 74 a concealed pinion 16 revolving in — of a lunation ; this 16 pinion has a short vertical arbor descending through a hole perforated in the monthly bar, and the small wheel 42, fast to the same, acts with the large contrate wheel 179, which has a horizontal arbor, pivoted into the end of a screw, at the lower end of a small cock fixed to the under face of the monthly bar. This part of the train is visible in the drawing, though the first portion is not. On the back face of the wheel 179, which revolves in the period of the moon's return to the node, is soldered an epicycloi dal rim of brass as an edge-bar, in an eccentrical position as it regards the wheel's arbor ; and a forked piece of steel, on which the lower end of the moon's squared portion of the stem rests, catches the epicycloidal edge-bar, which, during the wheel's motion, makes the remote end rise and fall alternately a proper quantity for giving the moon her due latitude, north or south of the ecliptic, in every posi tion of the earth, the centre of motion of the forked bar being a pin in the small cock into which the wheel's arbor is pivoted. According to these arrangements, the three mo tions of the moon are so united, as to produce the lunation, and the requisite variations of latitude, velocity, and dis tance, at the same tune; a union which has probably never before been so completely effected by such simple means. The broad edge of the wheel 179 has the graduations for the latitude, which arc indicated by the edge of the monthly bar. Above the trains on the monthly bar a large dial is fixed, along with the wheels 64 and 74 on the top of the cross bridge's tube; and a cranked index, similar to the an nual index, has a thread stretched over its open part to mark the day of the moon's age, and other graduations re lating to the tides, which may be inserted or omitted, ac cording to the wish of the maker or purchaser. This large dial has been considered as transparent in the draw ing, in order to show the situation of the dotted ovals, re presenting the concealed wheels and portions, which could not otherwise have been intelligibly described. The gra duations on the table are such as have reference to the sun, namely, a divided ecliptic, and corresponding circle of the sun's declination, with the day spaces laid down in due proportion and position, so that there are nearly eight days more in the summer than in the winter half-year; and yet all the circular motions and periods, connected with the indication by unequal divisions, are free from errors, not withstanding the earth's motion in its circuit round the sun has its actual velocity constantly varying. This appa rent paradox may be thus easily solved : the large wheel 269 has the size of its teeth varying from the aphelion to the perihelion points of the ecliptic, each way round, by this simple contrivance in the cutting and dividing of the teeth : a point was determined out of the centre of the wheel, which bore the same proportion to its radius that the earth's mean radius does to its eccentricity, which we believe in this wheel was two-tenths of an inch, and in this point a hole was drilled and broached, just large enough to admit the arbor of the cutting engine, and then the blank wheel placed upon this hole, as a centre, had its contrate teeth cut by the engine in the usual way, and the regular increase and decrease of the size of the teeth, throughout each respective semicircle, was thus mechanically insured, without any skill on the part of the operator, or powers of the engine, beyond what the prime number of teeth requir ed to he done during the common operation of cutting. This work, we understand, can be easily performed by Fayrer of Pentonville, near London, whose apparatus will enable him to cut a wheel into any number of teeth, prime as well as composite, and to round the teeth, without a file, at the same operation.
A globular lamp, of Argand's construction, is made the representative of the sun, and the rays of light are render ed parallel, or nearly so, by a couple of lenses, one of which is adjustable, as in a magic lantern; and by these means one half of the globe nearly is illuminated, while the other is in the shade, behind the semi-circular termi nator, and a lens, of small diameter and form, is interpos• cc] between the sun and earth, which, being properly ad justed, condenses the light it receives to a focus on the surface of the globe, the luminous point being just one hall of a degree in diameter. This luminous index shows
at all times the place where the sun is vertical; and the addition of adjustable meridian and horizon circles, pro p,,rly graduated, afford the means of working various pro blems connected with the sun's place, such as the time of his rising, setting, and culminating, his declination, the length of the day, and end of twilight, &c.; besides which, a small graduated quadrant applied at the zenith, and pas sing through the luminous point, which always represents the sun's place at any moment, (as seen from the centre of the earth,) will indicate his altitude, and show his azi muth on the horizon circle, at any hour of any clay in the year, so that the approximate solution of many interest ing spherical triangles may be performed in the most na tural way, from hour to hour, as the earth proceeds in her annual course, and at the same time turns round her in clined axis, which is always kept parallel to itself.
There is also a small lamp carried occasionally by the moon's stem, with a corresponding lens, to give another luminous point for describing the moon's path on the face of the earth which lens is kept in its parallel situation in every part of the month, and rises and falls together with its lamp, so as to show the variation of both latitude and longitude of the luminous point thus occasioned. If a celestial globe were substituted for the terrestrial, the moon's appulse to and passage over the zodiacal stars might thus be pleasingly represented, while both the solar and sidereal times would be pointed out, and nothing but the proper parallax of the moon would be wanting to pro duce the times (nearly) of the occultations.
There is, lastly, a contrivance, by means of a circular plate of brass, for projecting the moon's shadow upon the surface of the earth, to show on what part of the terres trial globe an eclipse of the sun may be expected to be vi sible; and a velocity is communicated to this plate by a lever, sufficient to make the shadow pass over the globe, large as it is, in the requisite time, or nearly so. In fact, the various problems usually worked on the terrestrial globe will be solved during the progress of the year, by the mere turning of the handle, when the meridian and hori zon circles are properly placed for the given longitude and latitude. The train at the handle, that gives motion to the weekly hand, resembles the dial-work of a common clock or watch ; the pinion 18 on the daily arbor drives an other 18, together with a smaller pinion of 8 leaves fast to it, and this latter again drives the wheel 56 fixed on, a tube, that revolves in 7 days round the arbor of the han dle, and its tube carries the weekly hand over the same dial that the hour hand points to, near the handle. 'When the annual index is required to be carried to a given day on the large graduated rim, the pinion on the long arbor is detached from the large wheel by its lifting screw, and the mechanism for preserving the parallelism is then all that remains in a state of action, in which situation the annual bar may be pushed round without without using the handle, and the change of seasons may thus be ex plained separately from the other phenomena. If the lu minous point is properly adjusted, and placed over the first degree of Aries on the globe, while the annual index points to the same, it will travel along and cover the ecliptic circle all round, as the annual bar is pushed round the table, and will thus show that the ecliptic circle itself is formed by uniting all the points where the sun is seen at noon of each successive day, into one continued circular line, which intersects the equator at an angle equal to the inclination of the earth's axis. At the same time it will appear, that the two poles of the earth are each presented to the sun's rays by turns, till the circle of illumination gradually extends to 234 degrees beyond the polar point, and then as gradually recedes, till the same appearance begins to take place at the opposite pole. Thus it may be explained, by means the most simple, viz. by a union of the naralletisnz with the inclination of the earth's axis, how all the beautiful variety in the face of nature, and the regular succession of bountiful supplies for the use of man and beast, are wonderfully produced by the power and wisdom of the omnipotent creator and preserver of the universe.