In this way the distance of a considerably remote object, as a town or building at 10 or 12 miles distant, may be very nearly determined; provided we have the lineal dimensions of a house or other object that stands at right angles to the line of vision. The breadth of a river, of an arm of the sea, or the distance of a light house, whose elevation above the sea or any other point, is known, may likewise in this manner be easily determined.
CHAPTER II.
ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY.
The equatorial instrument is intended to answer a number of useful purposes in Practical Astronomy, independently of any particular observatory. Besides answering the general purpose of a Quadrant, a Transit instrument, a Theodolite, and an Azimuth instrument—it is almost the only instrument adapted for viewing the stars and planets in the day-time, and for following them in their apparent diurnal motions. It may be made use of in any steady room or place, and performs most of the useful problems in astronomical science.
The basis of all equatorial instruments is a revolving axis, placed parallel to the axis of the earth, by which an attached telescope is made to follow a star or other celestial body in the arc of its diurnal revolution, without the trouble of repeated adjustments for changes of elevation, which quadrants and circles with vertical and horizontal axes require. Such an instrument is not only convenient for many useful and interesting purposes in celestial observations, but is essentially requisite in certain cases, particularly in examining and measuring the relative positions of two contiguous bodies, or in determining the diameters of the planets, when the spider’s-line micrometer is used.
Christopher Scheiner is supposed to have been the first astronomer who, in the year 1620, made use of a polar axis, but without any appendage of graduated circles. It was not, however, till the middle of the last century, that any instruments of this description, worthy of the name, were attempted to be constructed. In 1741, Mr. Henry Hindley, a clock-maker in York, added to the polar axis, an equatorial plate, a quadrant of altitude, and declination semicircle; but when this piece of mechanism was sent to London for sale in 1748, it remained unsold for the space of 13 years. Mr. Short, the optician, published in the Philosophical Transactions, for 1750, a ‘description of an equatorial telescope,’ which was of the reflecting kind, and was mounted over a combination of circles and semicircles, which were strong enough to support a tube, and a speculum of the Gregorian construction 18 inches in focal length. This instrument consisted of a somewhat cumbersome and expensive piece of machinery—a representation of which may be seen in volume III of Martin’s ‘Philosophia Britannica, or system of the Newtonian philosophy.’ Various modifications of this instrument have since been made by Nairne, Dollond, Ramsden, Troughton, and other artists; but even at the present period, it has never come into very general use, though it is one of the most pleasant and useful instruments connected with astronomical observations.
As many of these instruments are somewhat complicated, and very expensive, I shall direct the attention of the reader solely to one which I consider as the most simple—which may be purchased at a moderate expence, and is sufficiently accurate for general observations.
figure 86.
This instrument consists of the following parts: A horizontal circle EF (fig. 86.) divided into four quadrants of 90 degrees each. There is a fixed nonius at N; and the circle is capable of being turned round on an axis. In the centre of the horizontal circle is fixed a strong upright pillar, which supports the centre of a vertical semicircle AB, divided into two quadrants of 90 degrees each. This is called the semicircle of altitude, and may, at any time, serve the purpose of a quadrant in measuring either altitudes or depressions. It has a nonius plate at K. At right angles to the plane of this semicircle, the equatorial circle MN is firmly fixed. It represents the equator, and is divided into twice 12 hours, every hour being divided into 12 parts of 5 minutes each. Upon the equatorial circle moves another circle, with a chamfered edge, carrying a nonius by which the divisions on the equatorial may be read off to single minutes; and at right angles to this moveable circle is fixed the semicircle of declination D, divided into two quadrants of 90 degrees each. The telescope PO, is surmounted above this circle, and is fixed to an index moveable on the semicircle of declination, and carries a nonius opposite to Q. The telescope is furnished with 2 or 3 Huygenian eye-pieces, and likewise with a diagonal eye-piece for viewing objects near the zenith. Lastly, there are 2 spirit levels fixed on the horizontal circle, at right angles to each other, by means of which this circle is made perfectly level when observations are to be made.