The "watch" employed in the above observations was a sidereal clock with a dial shewing mean solar time. The error of the clock was known to be almost nil, but the indication of this fact by the sextant observations must only be regarded as a coincidence. However, the sextant in competent hands will generally give a result correct to within a few seconds.

The Box Sextant is merely a miniature sextant, very portable, and used rather for surveying than astronomical purposes, though, in conjunction with an artificial horizon, it becomes valuable for obtaining solar time, or the latitude.

The Prismatic Sextant, constructed by Pister and Martins of Berlin, differs from the common sextant, not only in its construction but in its capability, for it can measure angles up to

180°.8

Formerly the Snuff-box Sextant, from the limited space it occupies in transitu. Loomis, Pract. Ast., p. 101.

CHAPTER VI.

MISCELLANEOUS ASTRONOMICAL INSTRUMENTS.

The Altazimuth.-Everest's Theodolite.-The Mural Circle.-The Repeating Circle.— Troughton's Reflecting Circle.-The Dip-Sector.-The Zenith-Sector.-The Reflex Zenith-Tube. The Horizontal Floating Collimator. The Vertical Floating Collimator.-The Heliometer.-Airy's Orbit-Sweeper.-The Comet-Seeker.— The Astronomical Spectroscope.

THOUG

HOUGH the Equatorial and the Transit Instrument are the most important instruments used in astronomy, there are several others of an astronomical or semi-astronomical character, which should at least be glanced at in a work like the present. Still, as they are but rarely required by the amateur, my mention of them will be rather for the purpose of furnishing references to other works professedly devoted to their consideration than to treat of them with any fullness myself.

The following are the names of those I group under this head:1. The Altazimuth.

2. Everest's Theodolite.

3. The Mural Circle.

4. The Repeating Circle.

5. Troughton's Reflecting Circle.

6. The Dip-Sector.

7. The Zenith-Sector.

8. The Reflex Zenith-Tube.

9. The Horizontal Floating Collimator. 10. The Vertical Floating Collimator.

II. The Heliometer.

12. Airy's Orbit-Sweeper.

13. The Comet-Seeker.

14. The Astronomical Spectroscope.

The Altazimutha, as its name implies, is for the measurement

of altitudes and azimuths. It may be considered as a modification of the ordinary transit instrument - the telescope, circle, and stand of which are capable of motion round a vertical axis. The alta

a Pearson, Pract. Ast., vol. ii. pp. 413 457, 472; Heather, Mathematical Instruments, p. 153; Simms, Treatise on Instruments, p. 92; English Cyclopædia,

art. Astronomical Circle; Loomis, Pract. Ast., p. 93; Narrien, Ast. and Geod., P. 79.

zimuth may therefore be used for meridional or extra-meridional observations indifferently, and when of a portable size it may in fact be regarded as a theodolite of a superior construction. Fig. 214 represents such an instrument by Newton. In this form it is sometimes known under the name of the Transit Theodolite.

Fig. 215 is a pattern of a theodolite, known as "Everest's," from its designer, the late Sir G. Everest, Director of the Indian Survey. The arrangement adopted for the graduated arcs enables the observer, it is understood, to measure minute angles with greater

accuracy than is possible with an ordinary theodolite of corresponding size; and for geodetical purposes, especially in foreign countries, a maximum of precision, with a minimum of weight, is of course of the utmost importance. This instrument can be used within certain limits of altitude as an altazimuth b.

The Mural Circle consists of a graduated circle furnished with a suitable telescope, and very firmly affixed to a wall (murus) in the plane of the meridian. It is used for determining with

b Simms, Treatise on Instruments, p. 20.

great accuracy meridian altitudes and zenith distances, from which may be found declinations and polar distances.

The Repeating Circle is employed for the measurement of angular distances, both of celestial and terrestrial objects. The principle consists in repeating the readings of an angle several successive times and taking a mean, and thus eliminating almost wholly the errors due to defective graduation. Invented about the year 1744 by T. Mayer, this instrument was first constructed in France, under the superintendence of Borda, sometime between 1780 and 1790, and in that country it was much used. In England, however, it was never popular, firstly, because when it was invented the graduation of English instruments was so much superior to those of foreign make as to render it less needed; and secondly, because of the labour involved in working with it. Its value (theoretically very considerable) would seem to be impaired in practice by some defects which Sir J. Herschel, though he speaks of them as unknown, connects with imperfect clamping d.

Troughton's Reflecting Circle is a different adaptation of the principle involved in the sextant. It consists of a complete graduated circle, having the telescope and reflector on one side of the circle whilst the graduations and verniers (3 in number) are on the other. A reading being taken by each vernier, the mean of the three readings gives a more accurate result than would any one singly. In Sir J. Herschel's opinion "this is altogether a very refined and elegant instrument "."

The Dip-Sector, another instrument of Troughton's invention, is used for determining the dip of the horizon. The principle of it is similar to that of the sextant f.

The Zenith-Sector serves to determine the zenith point and the zenith distances of stars. It is, especially as modified by Airy, chiefly used in geodetical operations; but it was invented by Hooke

c Pearson, Pract. Ast., vol. ii. p. 472; English Cyclopædia, art. Astronomical Circle; Loomis, Pract. Ast., p. 83; Breen, Pract. Ast., p. 432; Narrien, Ast. and Geod., p. 76.

d Outlines of Ast., p. 119; Brewster's Cyclopædia, art. Circle; English Cyclopædia, art. Repeating Circle; Me

moirs R.A.S., vol. i. p. 33; Pearson, Pract. Ast., vol. ii. p. 578; Loomis, Pract. Ast., p. 103; Breen, Pract. Ast., p. 381.

e Pearson, Pract. Ast., vol. ii. p. 586; Simms, Treat. on Inst., p. 57; Heather, Math. Inst., p. 141.

Simms, Treat. on Inst., p. 65.