7i

Procure from an ironmonger twenty sash rollers, the larger the better, and let ten of them into the plate, one over each post; these are the bearing rollers : the other ten must do duty as guides to prevent the roof shifting laterally; they must be set into blocks of wood, with their pins vertical, and the blocks nailed to the plate, and also to the upper board. (See Fig. 7.)

Though this roof revolves well when the rollers are oiled, it is better to have a small tackle, consisting of two single blocks, called in the navy a “ gun tackle purchase;” the standing block being attached to the shutter frame, and a hook on the other block can be transferred successively to a screw eyebolt near the top of every other post inside, to pull the roof round.

A better but more expensive contrivance to revolve the roof consists of an endless iron rack, screwed to the ring, in which works a toothed wheel, with a strong pinion turned by a handle revolving on an axis fixed to the walls.

During the many years that observatories of this kind have stood, not one has had its roof blown off by a gale of wind. The force seems to act downwards on the conical surface : nevertheless, it is safest to have short lines, about one foot long, to lash it down to the eyebolts in one or two places.

The canvas should be well saturated with boiled linseed oil, and then have about three coats of paint, of the lightest gray colour, and the strips of wood and ornamentation, if any, round the eaves painted green.

The interior of the building will be finished by papering it on the canvas in the usual way; and any kind of light cornice upon the highest board adds to the internal appearance.

REFERENCES TO ILLUSTRATIONS.

Fig. 1.—South elevation.

Fig. 2.—Ground plan. A, A, A, &c., joists; P, pedestal of telescope.

Fig. 3.—Roof details.

Fig. 4.—Section of roof, N and S, showing the eccentricity of the ridge.

Fig. 5.—Section of ridge; ridge b; a, rafter; c, metal cover to ridge, attached to shutters.

Fig. 6.—Ridge, a, vertical section; b, horizontal section.

Fig. 7. —Upper end of a post; a, rafter ; c, plate ; e, weather board.

Partly from the English Mechanic.

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