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BERTHON’S PATENT EQUATORIALS.

The great increase in the number of amateur astronomers during the last few years, and the immense improvements in the optical parts of the instruments used by them, demand corresponding improvements in the mounting of Equatorial Telescopes.

Hitherto nearly all Equatorials belong to one of two types—the German and the English. The former is suitable for telescopes of moderate aperture; but with large sizes becomes expensive on account of the necessity of very perfect workmanship—the latter may be produced at less cost, but it is inconvenient, and has several disadvantages, though when expense is no object the defects may be reduced to a minimum by sundry elaborate contrivances, as in the large Achromatic in Greenwich Observatory.

The Equatorial Telescope Stands now to be described combine effectiveness with convenience at a very reduced expense. They are called the “ Equestrian, or Double Counterpoised Equatorial Stands,” and though adapted for all kinds of telescopes, they are particularly convenient for Newtonian reflectors.

The first form of these stands is shown in Fig. 22, and is suitable for telescopes of all sizes, but especially for those of about 6 or 8 in. in aperture. Its great peculiarity is the double counterpoises, one on each side of the polar axis. The body tube of the telescope revolves in a cradle, to which are attached two almost circular plates, whose centres have been turned to fit on the wide declination bearings 4 or 5 inches in diameter, which are attached to a hollow cylinder which revolves on a long polar axis attached to the firm iron foot. On each plate a counterpoise is screwed, and on the edge of one plate is a tangent screw by which a slow movement in declination can be applied. An endless screw gives a slow motion in right ascension, and can be worked with a tangent screw and Hook"'s joint. The declination arc is fixed to one of the plates, and the right ascension circle revolves on the cylinder. The great advantage of this form of stand is its great steadiness, all the bearings being several inches in diameter. The lower end of the body tube can be taken off and packed alongside the upper part, the whole of the working parts of the telescope can, therefore, be packed in a comparatively small case, and the heavy pedestal carried alone. As it will not suffer from exposure, the pedestal can be left in equatorial position out of doors, and the whole of the telescope, counterpoise, movements, &c., can be carried to a safe place in one piece.

. The principle of the other equatorial devised by Mr. Berthon is the same as that just described, but its construction is different. It is represented in Fig. 23. A well-turned disc of iron, 16 or more inches in diameter, is attached to an iron pedestal at an angle equal to that of the co-latitude of the place for- which the telescope is intended, and upon this another disc revolves, being kept from sliding off by a central pivot, or polar axis. This upper disc carries the telescope,

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