instrument between the observations, and if any, it immediately detected them. This was also a severe check upon the divisions of the instrument. General Ray observes, that it rarely happens that two observers, reading off with the opposite microscopes, differ more than half a second from each other at the first reading; and judges, that in favourable weather for repeating the observation with the telescope, a wonderful degree of accuracy in the measure of the angles may be obtained.*

For the auxiliary apparatus, such as the 100 feet steel chain, portable scaffold, tripod ladder, common flag-staff, tripod for white lights, portable crane, &c. the reader will see the account of in the Transactions before cited. The horizontal angles taken by the instrument as regulated by the General, since deceased, are to the tenth of a second.

* The weight of the whole instrument was about 200 pounds, and the price, as I have been informed, about 350 guineas. By the completion of the measurements and the necessary calculations, the difference of the two meridians made 9′ 20′′, as before fixed by Dr Maskelyne.

A second instrument has since been made, and is now using by Col. Williams, Capt. Mudge, and Mr. Dalby, from whose skill and ingenuity it is expected a very accurate survey of this country will be made. In this instrument, the great circle is divided to 10 minutes, improvements made in the microscopes, &c. by Mr. Ramsden. See Philosophical Transactions for 1795.

A separate and complete account of this Trigonometrical Survey of England and Wales has been recently published by Col. Mudge and Mr. Dalby, in 3 vols. 4to. 1799-1811.

DESCRIPTION, USE,

AND METHOD OF ADJUSTING

HADLEY'S QUADRANT.*

"Ar the appointed time, when it pleased the Supreme Dispenser of every good gift to restore light to a bewildered world, and more particularly to manifest his wisdom in the simplicity, as well as in the grandeur of his works, he opened the glorious scene with the revival of sound astronomy." This observation of an excellent philosopher and physician is verified in every instance of the progress of science; in each of which we may trace some of the steps of that vast plan of Divine Providence to which all things are converging, namely, the bringing all his creatures to a state of truth, goodness, and consequent happiness; an end worthy of the best and wisest of beings, and which we may perceive to be gradually effecting, by the advancement of knowledge, the diffusion of liberty, and the removal of error, that truth and virtue may at last shine forth in all the beauty of their native colours.

It is thus that the discovery of the compass gave rise to the present art of navigation; and when this art grew of more importance to mankind, Divine Providence blessed them with the invention of Hadley's Quadrant, and in our own day and in our own time has further improved both it and the art of navigation, by the present method of finding the longitude, which enables the mariner to ascertain with certainty his situation on the unvaried face of the ocean.

This account of Hadley's quadrant, &c. is extracted from a small tract I published thereon sometime since, 8vo. Sir John Pringle's Six Discourses to the Royal Society. R

Hadley's quadrant or sextant is the only known instrument, on which the mariner can depend for determining with accuracy and precision his latitude and longitude. It is to the use of this instrument that navigation is indebted for the very great and rapid advances it has made within these few years. It is easy to manage, and of extensive use, requiring no peculiar steadiness of hand, nor any such fixed basis as is necessary to other astronomical instruments. It is not the science of navigation only which is indebted to this instrument, but its uses are so extensive in astronomy, that it may, accompanied with an artificial horizon, with propriety be called a portable observatory, and in this work we shall exemplify its application to surveying.

Mankind are ever desirous of knowing to whom they are indebted for any peculiar or useful discovery; it is the tribute of gratitude, and a reward to merit. In the present instance there is no difficulty in giving the information; the respective claims of the inventors are easily decided. The first thought originated with the celebrated Dr. Hooke, it was completed by Sir Isaac Newton, and published by Mr. Hadley.

Notwithstanding, however, the manifest superiority of this instrument over those that were in use at the time of its publication, it was many years before the sailors could be persuaded to adopt it, and lay aside their imperfect and inaccurate instruments: so great is the difficulty to remove prejudice, and emancipate the mind from the slavery of opinion.

No instrument has undergone, since the original invention, more changes than the quadrant of Hadley; of the various alterations, many have had no better foundation than the conceit and caprice of the makers, who, by these attempts, have often rendered the instrument more complicated in construetion and more difficult in use, than it was in its original state.

ESSENTIAL PROPERTIES OF HADLEY'S QUADRANT.

It is not my intention under this head to enumerate all the advantages of this instrument; but bare ly to point out one or two of those essential properties which distinguish it from every other instrument of the kind, and rank it among one of the greatest improvements in the practice of navigation.

It is an essential property of this instrument, derived from the laws of reflection, that half degrees on the arc answer to whole ones in the angles measured; hence an octant, or the eighth part of a cir cle, or 45 degrees on the arc, serves to measure 90 degrees; and sextants will measure an angular distance of 120 degrees, though the arc of the instrument is no more than 60 degrees.* It is from this property that foreigners term that instrument an octant, which we usually call a quadrant, and which in effect it is. The property reduces indeed considerably the bulk of the instrument; but at the same time it calls for the utmost accuracy in the divisions, as every error on the arc is doubled in the obser vation.

Another essential, and indeed an invaluable property of this instrument, whereby it is rendered peculiarly advantageous in marine observations, is, that it does not require any peculiar steadiness of the hand, nor is liable to be disturbed by the ship's motion; for, provided the mariner can see distinctly the two objects in the field of his instrument, no motion, nor vacillation of the ship, will hinder his observation.

Thirdly, the errors to which it is liable are easily discovered, and readily rectified, while the applica tion and use of it is facile and plain.

The principal requisites in a good sextant or quad

*For a concise explanation of the theory, &c. see my pain. phlet on this instrument, 8vo. 1808

EDIT.

rant, are, 1. That it be strong, and so constructed as not to bend across the plain. 2. That it be accurately divided. 3. That the surfaces of the glasses be perfectly plain and parallel to each other. 4. That the index turn upon a long axis. 5. That the motion be free and easy in every part, and yet without the least shake or jerk.

DESCRIPTION OF HADLEY'S QUADRANT.

Fig. 1, plate 19, represents a quadrant, or octant, of the common construction. The following parts are those which require the particular attention of the observer.

I.

II.

III.

IV.

V.

BC the arc.

AD the index, a b the nonius scale.

E the index-glass.

F the fore horizon-glass.

G the back horizon-glass.

VI. K the dark glasses or screens.

VII. HI the vanes or sights.

VIII. The arc BC is called the limb or quadrantal arc; the arc cd lying from o, towards the right is called the arc of

excess.

OF THE QUADRANT.

The quadrant consists of an arc BC, firmly attached to two radii, or bars, AB, AC, which are strengthened and bound together by the two braces LM.

Of the index. The index D is a flat bar of brass, that turns on the centre of the octant; at the lower end of the index there is an oblong opening, to one side of this opening a nonius scale is fixed to subdivide the divisions of the arc; at the bottom or end of the index there is a piece of brass, which bends under the arc, carrying a spring to make the