Sully, at the end of the letter of which an extract has just been given, makes the following remarks, page 9 : What the Rev. Father Kresa relates of the clock of the late King of Spain, is very true. It is more than twenty years since such clocks were made in London, and I believe that I am the first who applied this mechanism (for equation) to a pocket watch, twelve or fourteen years ago." The following is a description of a very excellent and curious equation clock, which belonged to the late Gene ral Clerk. It was left, with several other things, to the late Sir John Clerk, and entailed on the house of Penny cuik.
The clock goes a month, strikes the hour, and has a strike silent piece. The 'scapement of it is made after that of John Harrison's, requiring no oil to the pallets ; (see p. 476.) and the pendulum is a gridiron compensation one, composed of five rods, three of which are steel, the other two of zinc, or some compound of zinc. On the dial are seen the hours, minutes, and seconds, and their hands. The minute hand keeps mean or equal time ; the equation of time is given by a hand with a figure of the sun on it, which makes a revolution every hour as the minute hand does, only for the most part it goes sometimes a very little slower, and sometimes a little faster than the minute hand, keeping solar or unequal time, and shows at all times when the sun is on the meridian. The age and phases of the moon are also represented, the days of the year and of the month, the degrees of the ecliptic, and the signs of the zodiac, the rising and setting of the sun, the length of the day, &c. The dial is a twelve inch arched one. Con centric with the arch is a sort of ring plate wheel of 365 teeth, making its revolution in a year, or 365 days. Its di ameter is about 8 inches, and the breadth of the rim or ring 11 inch nearly. On this ring plate, at the outermost circles containing divisions, are laid down the days of the year ; and on the space next within, are the names of the months, the days being numbered by the figures 10, 20, 30, and so on. The next circles contain the 360 degrees of the ecliptic ; the space within has the signs of the zodiac, and the numbers 10, 20, and 30, marked for the degrees in every sign, and corresponding with the days of the year and of the month when the sun is in any of these signs. The innermost circles contain what may be called the divi sions of the semi•diurnal arcs. On the space outside of this, are marked the corresponding hour figures in Roman characters. This is what gives the time of the rising and setting of the sun, and the length of the day.
In the annual plate ring, are rivetted six small brass pil lars, one inch and one tenth of an inch in height, whose op posite ends are screwed by steel screws, and their heads sunk into a plain ring wheel neatly crossed into six arms, the diameter being five inches and three quarters of an inch, and the breadth of the rim three-eighths of an inch. The back of this plain ring is distant from the back of the annual plate ring one inch and a quarter. The plain ring is at the centre, screwed on a brass socket, having a square hole in it. Within the frame of the clock movement, and at a perpendicular distance of six inches from the centre wheel holes, a steel arbor is run in, and at one end pro longed about an inch and a half beyond the fore frame plate, somewhat like a stud. The pivot in the fore plate is
of such a length and thickness, as to allow a square on its outside. It is on this square that the equation elliptic plate is put, and above it is put on the annual plane ring by means of its socket, with a square hole in it. That part of the arbor which is above this socket is round, and serves as a stud for the moon's age ring socket, to revolve on it freely -Ind easily. The moon's age ring turns within the annual plate ring, and is divided into 59 equal parts, numbered q, 6, 9, and so on to 291. Its diameter is live inches an d one eighth ; its breadth fully three-eighths of an inch, and it is connected with a plain wheel neatly crossed into six arms, of the same diameter and breadth of rim as the moon's age ring, having six small pillars, nearly an inch in height, rivetted into it, and the moon's age ring screwed at the op posite ends of three of them, by three sunk steel screws. This plain ring has a socket, which runs or turns on the stud, above the annual plain wheel ; the face of the moon's age ring comes flush up with that of the annual plate ring, and both come up to the back of the dial, in which an open ing in the arch is made, in older to shew a great part of what is on these rings. From the top of the arch, across the opening, and down in a straight direction, is stretched a very fine wire, serving as an index to the days of the year, the moon's age, &c. The annual plate ring and the moon's age ring move or turn from the right to the left hand, yet separately and independently of each other. On the inside shoulder of the socket of the moon's age ring is screwed a small bevelled wheel, having 37 teeth, and one inch in diameter, the use of which will be afterwards ex plained. In the dial is a circular opening of one inch and three quarters in diameter, a little below the opening in the arch ; in this opening is exhibited the lunar globe of an inch and a quarter in diameter, made of brass and silvered; one half of it is perpendicularly painted black, in order to give the phases, the new and the full moon. On the arbor of the lunar globe are two wheels, one of 63 teeth, and about an inch in diameter, the other a bevelled one of the same diameter, and with 37 teeth. Both are placed below the lunar globe ; the wheel of 37 and the globe arc fast on the arbor, the wheel of 63 being keyed spring-tight above the bevelled one. The arbor of the lunar globe is in the plane of the dial or nearly so, and this bevelled wheel takes or pitches into that of the same number, which is screwed on the moon's age ring socket, as before mentioned ; and by means of holes in the ring, the whole, that is, the globe, the bevelled wheels, and moon's age ring, Etc. can be made to turn together, when the moon is at any time setting to its proper age. The pivots of the lunar globe arbor run on cocks which are screwed on to the back of the dial. Behind the globe, and at a little distance from it, is screw ed on to the hack of the dial, a sort of concave or hollow hemisphere of thin plate brass, painted inside of a sky blue colour.