is so adjusted, that 15 revolutions of the head shall move the wire over 15', or one grand division of the limb, equal to 900", each degree on the circle being only divided into four parts. To effect this, at the same time the fixed wire must bisect the dot on a gold tongue, the moveable wire must also bisect the dot at 180° on the limb as well as a first notch in the magnified scale at the bottom of the plate. In this adjustment there is another circumstance to be attended to, viz. that sixty on the micrometer head should stand nearly vertical, so as to be conveniently seen; a few seconds of inclination are of no consequence, because the dart or index being brought to that position, whatever it may be, must always remain there, any derangement of the instrument excepted. But if, when the wires coincide with their respective dots, and the first notch 60 on the micrometer head should happen to be underneath, or so far from the vertex side as to be seen with difficulty, then the gold tongue is to be moved a little by capstan head screws, which act against each other on the opposite extremities of the axis. Thus, by repeated trials the purpose will be effected, viz. the 60, to which the dart is to be set, will stand in a place easily seen. It is not to be expected that each microscope will give just 900 seconds for the run of 15 minutes; without loss of time this cannot be done besides, two observers will adjust the microscopes differently. After several trials of the runs in measuring 15 minutes in the different parts of the limb, one microscope, gave 896", while the other gave at a medium 901"; in a year afterwards, the former gave 900", while the latter gave 894". These differences were allowed for in the estimation of angles for computation.

The gold tongue mentioned is extremely thin, and goes close to the surface of the circle. This contrivance of a tongue with a dot was to guard against any error from any accidental motion given to the

vertical microscopes S, S. The arms, braces, base, &c. are every where pierced, in order to lessen weight without diminishing strength.

The angles are not read off in this instrument by a nonius as common to others, but with microscopes, which form the most essential part of the instrument. But a short account of them can be given here, an adequate idea can only be obtained by a reference to the Philosophical Transactions, page 145 and 149. That horizontal microscope for the vertical angles has been already mentioned. The two vertical ones S, S, arc used for reading off the divisions on the opposite sides of the circle immediately under them. Each microscope contains two slides, one over the other, their contiguous surfaces in the foci of the eye glasses. The upper one is a very thin brass plate; at its lower surface is attached a fixed wire, having no other motion than what is necessary for adjustment, by the left hand screw to its proper dot, as hereafter to be explained. The other slide is of steel, of one entire piece, directly under the former, of sufficient thickness to permit a micrometer screw of about 72 threads in an inch to be formed of it. To its upper surface is fixed the immoveable wire, which changes its place by the motion of the micrometer head. This head is divided into 60 equal parts, each of which represents one second or angular motion of the telescope. This steel slide is attached by a chain to the spring of a watch coiled up within a small barrel adjacent to it in the frame. By this no time whatever is lost, the smallest motion of the head being instantly shewn by a proportionable motion of the wire in the field of the microscope.

Each microscope is supported between its pillars, and can be a little raised or depressed in respect to the plain of the circle by two levers. By this motion distinctness is obtained of the wires, and by the motion of the proper screw of the object lens, which follows that given to the whole microscope, the scale

is so adjusted, that 15 revolutions of the head shall move the wire over 15', or one grand division of the limb, equal to 900", each degree on the circle being only divided into four parts. To effect this, at the same time the fixed wire must bisect the dot on a gold tongue, the moveable wire must also bisect the dot at 180° on the limb as well as a first notch in the magnified scale at the bottom of the plate. In this adjustment there is another circumstance to be attended to, viz. that sixty on the micrometer head should stand nearly vertical, so as to be conveniently seen; a few seconds of inclination are of no consequence, because the dart or index being brought to that position, whatever it may be, must always remain there, any derangement of the instrument excepted. But if, when the wires coincide with their respective dots, and the first notch 60 on the micrometer head should happen to be underneath, or so far from the vertex side as to be seen with difficulty, then the gold tongue is to be moved a little by capstan head screws, which act against each other on the opposite extremities of the axis. Thus, by repeated trials the purpose will be effected, viz. the 60, to which the dart is to be set, will stand in a place easily seen. It is not to be expected that each microscope will give just 900 seconds for the run of 15 minutes; without loss of time this cannot be done besides, two observers will adjust the microscopes differently. After several trials of the runs in measuring 15 minutes in the different parts of the limb, one microscope, gave 896", while the other gave at a medium 901"; in a year afterwards, the former gave 900", while the latter gave 894". These differences were allowed for in the estimation of angles for computation.

The gold tongue mentioned is extremely thin, and goes close to the surface of the circle. This contrivance of a tongue with a dot was to guard against any error from any accidental motion given to the

instrument between the observations, and if any, it immediately detected them. This was also a severe check upon the divisions of the instrument. General Roy 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.*

This

"AT 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." 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.