as formerly described. This horizontal bar being fixed to the top of the tube of the hollow axis of the 4-inch pulley, must with it move round in the same time, and with it the Moon, causing it to revolve round the Earth in 29 days 12 hours 45 minutes, as indicated by the passage of the annual index over the days in the large ecliptic-circle, or 12,5% revolutions round the Earth in the time taken by the annual index in traversing all the signs and degrees of the ecliptic. The Moon is half covered with a black cap, which, by means of a bent wire, is connected with a swivel-pin on the top of the Sun; this pin has a small hole in it, through which the end of the long wire has a lateral motion to accommodate the variable distances of the Moon from the Sun in its revolution round the Earth; the other end of this wire being fixed to the back of the Moon's cap, causes the front of it to look always towards the Sun during its circuit round the Earth, thereby showing the varied phases of the Moon as seen from the Earth-and always a full Moon as seen from the Sun. Above the 4-inch pulley there is another one, G, having a sharp-bottomed groove, the diameter of which, measured from the bottom of the groove, must be 12 inch nearly; this pulley is, by means of an uncrossed cat-gut string, connected with the upper central pulley L, which, as before noted, is exactly 2 inches in diameter, as measured from the bottom of its groove. This pulley being 6th of an inch less than that in the centre, will not keep its parallelism as the lower pulley E does, but will gain a little yearly, giving a retrograde motion to it in 183 years, which is the period of the Moon's nodes. Instead of a pulley 1 inch in diameter at the bottom of its groove, one of will give a more correct nodal revolution; but as it is impossible to make a groove to answer this fraction, the bottom of the groove ought to be made 1 inch in diameter, and then by trials, nicely reduced a little, by means of a triangularshaped file. This pulley G has a hollow axis which turns on that of the Moon, and ascends about 1 inch above the surface of the box, and carries on its top the oblique orbit of the Moon N, and causes it to make a retrograde revolution through all the signs and degrees of the ecliptic in 183 years. In concluding our description of the internal arrangements of this very simple orrery, we may note that the grooves in all the pulleys ought to be shaped like the letter V, sharp at the bottom, in order that the cat-gut strings may not slip during the motion of the box. That a greater degree of security may be given to these cat-gut strings to prevent them slipping, we would recommend that each of the strings, at the letters H I J, be furnished with a pulley moving on a joint, and by means of a spring, cause the pulley to press the several cat-gut strings inwards; this would keep the strings tight on the pulleys in all weather and under all circumstances. As noticed at the beginning of the description, Ferguson mentions that he improved this machine soon after it was made "by adding another wheel" to it, and then called it "the Calculator." Whether Ferguson then made an entirely new machine by adding an extra wheel to it, or simply altered the four-wheeled orrery into the Calculator, is now not known. If he altered the four-wheeled orrery into the Calculator, he would do so by placing the pulleys into a new frame and bringing them into closer contact, as shown in the pulley-work of the Calculator; and in having the works concealed from view by a circular cover-plate, moving with the annual motion. (See view of the Calculator, under date 1749). It is singular that Ferguson should call this machine a four-wheeled orrery; a four-pulleyed, or rather a six-pulleyed orrery would have been the more correct designation; because, the central mass does not act as one, but as three distinct pulleys. Ferguson calls these pulleys, wheels. Wheels are usually understood to have teeth or cogs, while pulleys are wheels without teeth, simply grooved or plain round the circumference. 1747. "THE IMPROVED CELESTIAL GLOBE."-About the beginning of the year 1747, Ferguson improved the Celestial Globe by adding a piece of apparatus to it which enabled him to show the apparent diurnal revolutions of the Sun and Moon, with the times of their rising, culminating, and setting, the age of the Moon, &c. This improved globe was exhibited before the members of the Royal Society, and at same time, there was read a paper, written by Ferguson, descriptive of the new improvement and its use. The paper and the improved globe were approved of by the members, and the description was ordered to be printed, which was done in due course. The paper stands indexed in their Transactions as, "An Improvement of the Celestial Globe, by James Fer- We annex an engraving of the Celestial Globe, as improved by Ferguson, along with a full description of it. "On the north pole of the axis, above the hour circle, is fixed an arc M KH of 23 degrees, and at the end H is fixed an upright pin HG, which stands directly over the north pole of the ecliptic, and perpendicular to that part of the surface of the globe. On this pin are two moveable collets at D and H, to which are fixed the quadrantal wires N and O, having two little balls on their ends for the Sun and Moon, as in the figure. The collet D is fixed to the circular plate F, on which the 29 days of the Moon's age are engraven, beginning just under the Sun's wire N; and as this wire is moved round the globe the plate F turns round with it. These wires are easily turned if the screw G be slackened; and when they are set to their proper places, the screw serves to fix them there, so that when the globe is turned, the wires with the Sun and Moon may go round with it; and these two little balls rise and set at the same times and on the same points of the horizon, for the day to which they are rectified, as the Sun and Moon do in the heavens. Because the Moon keeps not in her course in the ecliptic (as the Sun appears to do), but has a declination of 5 degrees on each side from it in every lunation, her ball may be screwed as many degrees to either side of the ecliptic as her latitude or declination from the ecliptic amounts to at any given time; and for this purpose, S is a small piece of pasteboard, of which the curved edge at S is to be set upon the globe at right angles to the ecliptic, and the dark line over S to stand upright upon it. From this line, on the convex edge, are drawn 5 degrees of the Moon's latitude on both sides of the ecliptic; and when this piece is set upright on the globe, its graduated edge reaches to the Moon on the wire O, by which means she is easily adjusted to her latitude, found by an ephemeris. The horizon is supported by two semicircular arcs, because pillars would stop the progress of the balls when they go below the horizon in an oblique sphere. 147 To rectify this Globe.-Elevate the pole to the latitude of the place; then bring the Sun's place to the ecliptic for the given day to the brass meridian, and set the hour-index to XII. at noon, that is, to the upper XII. on the hour-circle, keeping the globe in that situation; slacken the screw G and set the Sun directly over his place on the meridian, which being done, set the Moon's wire under the number that expresses her age for that day on the plate F, and she will then stand over her place in the ecliptic and show what constellation she is in; lastly, fasten the screw G, and laying the curved edge of the pasteboard, S, over the ecliptic, below the Moon, adjust the Moon to her latitude over the graduated edge of the pasteboard, and the globe will be rectified. Its use. Having thus rectified the globe, turn it round, and observe on what points of the horizon the Sun and Moon balls 147 This simple auxiliary apparatus of Ferguson's may be used with effect by those who have their Celestial Globe mounted on a pillar. We had the apparatus made for our globe, and the cost was only 30s.; and we believe that the same mechanician continues to supply them at same price. rise and set, for these agree with the points of the compass on which the Sun and Moon rise and set in the heavens on the given day; and the hour-index shows the times of their rising and setting, and likewise the time of the Moon's passage over the meridian. This simple apparatus shows all the varieties that can happen in the rising of the Sun and Moon, and makes the phenomena of the Harvest Moon plain to the eye. It is also very useful in reading lectures on the globes, because a large company can see this Sun and Moon go round, rising above and setting below the horizon at different times, according to the seasons of the year, and making their appulses to different fixed stars. But in the usual way, where there is only the places of the Sun and Moon in the ecliptic to keep the eye upon, they are easily lost sight of, unless they be covered with patches." (See Ferguson's Astronomy, Chapter XII. Astronomical Machinery, article 401, and Plate III., figure 3; also, The Universal Magazine, Vol. 7, with Plate, &c.) 'A DISSERTATION UPON THE PHENOMENA OF THE HARVEST MOON," &c., Published.-Ferguson published this his second work in July, 1747, being a small 8vo of 72 pages, and illustrated by three folding engravings. As formerly mentioned, Ferguson, in his own memoir, notices this publication, and calls it his first work; but it was really his second. At page 5 of his preface to this "Dissertation," he refers to his first workapologises for an error in it, and also for the printer, for his faulty printing. We do not find "the Dissertation" advertised in any of the London newspapers or magazines before July, 1747; and as it comes first into notice this month, we conclude it was then just published. The following is a copy of an advertisement of it in the Gentleman's Magazine :-" Dissertation on the Phenomena of the Harrest Moon; 3 Copper-plates; by James Ferguson." (Gentleman's Magazine, Vol. 17, p. 348, July, 1747; see also note 82.) THE PLANETARY GLOBES INSTRUMENT-During the summer of 1747, Ferguson invented and made, for the illustration of his future lectures, a curious and simple apparatus, which he afterwards designated "THE PLANETARY GLOBES." The annexed |