SECTION II.

Description of the Instruments requisite in Astronomica?

Observations.

THE QUADRANT.

It is generally allowed that we are indebted to

T John Hadley, Esq. for the invention, or at least for the first public account of that admirable instrument, commonly called Hadley's Quadrant, who in the year 1731, first communicated its principles to the Royal Society, which were by them published soon after in their Philosophical Transactions; before this period, the Cross Staff and Davis's Quadrant were the only instruments used for measuring altitudes at sea, both very imperfect, and liable to considerable error in rough wea ther; the superior excellence however of Hadley's Quadrant, soon obtained its general use among seamen, and the many improvements this instrument has received from ingenious men at various times, has rendered it so correct, that it is now applied, with the greatest success, to the important purposes of ascertaining both the latitude and longitude at sea, or land.

The Octant or Frame, is generally made of ebony, or other hard wood, and consists of an arch firmly attached to two radii, or bars, which are strengthened and bound by the two braces, in or der to prevent it from warping.

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The Arch, or Limb, although only the eighth part of a circle, is on account of the double reflection, divided into 90 degrees, numbered 0, 10, 20, 30, &c. from the right towards the left; these are subdivided into 3 parts, containing each 20 minutes, which are again subdivided into single minutes, by means of a scale at the end of the Index. The arch extending from 0 towards the right hand is called the arch of excess.

The Index is a flat brass bar, that turns on the centre of the instrument; 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 arch; at the bottom or end of the index, there is a piece of brass which bends under the arch, carrying a spring to make the Nonius scale lie close to the divisions; it is also furnished with a screw to fix the Index in any desired position.

Some instruments have an adjusting or tangentscrew, fitted to the Index, that it may be moved more slowly, and with greater regularity and accuracy than by the hand; it is proper, however, to observe, that the Index must be previously fixéd near its right position by the above mentioned screw, before the adjusting screw is put in motion.

The Nonius is a scale fixed to the end of the Index for the purpose, as before observed, of dividing the subdivisions on the Arch into Minutes; it sometimes contains a space of 7 degrees, or 21 subdivisions of the limb, and is divided into 20 equal parts; hence each division on the Nonius will be one-twentieth part greater, that is, one miAute longer than the divisions on the Arch; con

sequently, if the first division of the Nonius marked 0, be set precisely opposite to any degree, the relative position of the Nonius and the Arch must. be altered one minute before the next division on the Nonius will coincide with the next division on the Arch, the second division will require a change of 2 minutes, the third of 3 minutes, and so on, till the 20th stroke on the Nonius arrives at the next 20 minutes on the Arch; the 0 on the Nonius will then have moved exactly 20 minutes from the division whence it set out, and the intermediate divisions of each minute, have been regularly pointed out by the divisions of the Nonius.

The divisions of the Nonius scale are in the above case reckoned from the middle towards the right, and from the left towards the middle; therefore the first 10 minutes are contained on the right of the 0, and the other 10 on the left. But this method of reckoning the divisions being found inconvenient, they are more generally counted, beginning from the right-hand towards the left; and then 20 divisions on the Nonius are equal to 19 on the limb, consequently one division on the Arch will exceed one on the Nonius by one-twentieth part, that is, one minute.

The 0 on the Nonius, points out the entire de grees and odd twenty minutes subtended by the objects observed; and if it coincides with a division on the Arch, points out the required angle: thus, suppose the 0 on the Nonius stands at 25 degrees, then 25 degrees will be the measure of the angles observed; if it coincides with the next division on the left hand, 25 degrees 20 minutes is the angle; if with the second division beyond 25

degrees, then the angle will be 25 degrees 40 minutes; and so on in every instance where the 0 on the Nonius coincides with a division on the Arch; but if it does not coincide, then look for a division on the Nonius that stands directly opposite to one on the Arch, and that division on the Nonius gives the odd minutes to be added to that on the Arch nearest the right-hand of the 0 on the Nonius; for example, suppose the Index division does not coincide with 25 degrees, but that the next division to it on the Nonius is the first coincident division, then is the required Angle 25 degrees 1 minute; if it had been the second division, the Angle would have been 25 degrees 2 minutes, and so on to 20 minutes, when the 0 on the Nonius would coincide with the first 20 minutes on the Arch from 25 degrees. Again, let us suppose the 0 on the Nonius to stand between 50 degrees and 50 degrees 20 minutes, and that the 15th division on the Nonius coincides with a division on the Arch, then is the angle 50 degrees 15 minutes. Further, let the 0 on the Nonius stand between> 45 degrees 20 minutes and 45 degrees 40 minutes, and at the same time, the 14th division on the Nonius stands directly opposite to a division on the Arch, then will the Angle be 45 degrees 34 mi

nutes.

The Index Glass is a plane speculum, or mirror of glass quicksilvered, set in a brass frame, and so placed that the face of it is perpendicular to the plane of the instrument, and immediately over the centre of motion of the Index. This mirror being fixed to the Index moves along with it, and has its direction changed by the motion thereof.

This glass is designed to reflect the image of the Sun, or any other object, upon either of the two horizon glasses, from whence it is reflected to the

eye of the observer. The brass frame, with the glass, is fixed to the Index by the screw; the other screw serves to place it in a perpendicular posi tion, if by any accident it has been put out of or der.

The Horizon Glasses are two small speculums on the radius of the Octant; the surface of the upper one is parallel to the Index glass when the 0 on the Nonius is at 0 on the Arch; these mirrors receive the rays of the object reflected from the Index glass, and transmit them to the observer, The fore Horizon glass is only silvered on its lower half, the upper half being transparent, in order that the direct object may be seen through it. The back Horizon glass is silvered at both ends; in the middle there is a transparent slit, through which the Horizon may be seen. Each of these

glasses is set in a brass frame, to which there is an axis; this axis passes through the wood work, and is fitted to a lever on the under side of the quadrant, by which the glass may be turned a few degrees on its axis, in order to set it parallel to the Index glass.

To set the glasses perpendicular to the plane of the quadrant, there are two sunk screws, one before and one behind each glass: these screws pass through the plate on which the frame is fixed into another plate, so that by loosening one and tightening the other of these screws, the direction of the frame, with its mirror, may be altered, and thus be set perpendicular to the plane of the instrument.

The Dark Glasses, or Shades, are used to prevent the bright rays of the Sun, or the glare of the Moon, from hurting the eye at the time of observation; there are generally three of them, two red, and one green. They are each set in a brass frame

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