We will now suppose an observer equipped with such a sextant, who wishes to make the best use of it.. Before taking a series of observa tions, the index error should lie ascertained. The telescope is fixed and set to focus, and the parallel wires placed parallel with the plane of the instrument. If the sun is visible, a dark glass for the eye-end of the telescope is selected, which shows him clearly, but without distressing the eye, and the direct and reflected images are made equally bright by moving the adjusting screw, which raises or depresses the telescope. The contact is first made, bringing the reflected image apparently below the image seen directly, and the angle is read off on the supplemental or back arc : suppose this reading to be 33', which consider positive or +. Then make the contact again with the reflected image apparently above that seen directly (the reflected image is that which moves on moving the index), and read off again : suppose this is 32', which call —. It is evident that at the middle position the two images coincide, and the index and horizon-glass are parallel; that is, the reading on the back arc is 30" when the glasses are parallel. Hence 30" must be added to every other reading to give the true angle. If the algebraical signs are used, the rule is very simple : Add the two readings together and halve the result ; this with its proper sign is the index correction to be applied to all observations. If the sun is not visible, the moon may be used in the same way. If neither is to be seen, then bring the two images of a bright star, or the sea horizon, or any distant well-defined object into contact, and the reading, if on the back arc, is to be added, if on the forward arc, is to be subtracted from all other observations. The sun is always to be preferred when visible, and the observation is to be repeated before or after correction. If the images overlap laterally, that is, if the horizon-glass is not parallel to the index-glass, this must be previously adjusted. The next caution is with respect to dark glasses. When it is possible (as in observing altitudes of the sun at land in a mercurial horizon, &c.) to make the observaticrn with a single dark glass on the eye-end of the telescope, without using any shade, this should always be done, for the error of this dark pies doea not affect the contact at all, and the distortion caused by it is not magnified, whereas any fault in the dark shade between the index and horizon glasses produces actual error in the observation, and the distortion is magnified subse quently by the telescope. The images are to be brought to equal brightness by the screw which raises or depresses the collar carrying the telescope, and that with considerable nicety. In observing distances of the sun and moon, or altitudes of the sun at sea, dark shades are necessary. The fainter object is to be observed directly, and a proper shade interposed between the index and horizon glasses to reduce the two objects to something like the same brightness. The final equalisation is to be effected by the up-and-down motion of the telescope. The accuracy of the observation depends a good deal upon the nicety with which this equal toning of the two images is effected : with a little experience this is readily learned. From the construction of the sextant the faint object is easily viewed directly when the brighter object is either above or on the right hand of the fainter, but not so if the bright object is to the left of the fainter, when the handle side of the instrument is uppermost. For these observations, the Instrument should have a second handle, which may be applied when wanted; perhaps when the distance between the moon and a bright star or planet is to be observed, there may be no difficulty in viewing the moon directly, after placing a dark shade before the horizon-glass, though the light which enters the eye without paaaing through the telescope must deaden the retina considerably. A blackened card screen slipped over the tube of the telescope would probably be found useful in this case, and absolutely necessary if the sun were observed directly. This latter observation cannot, we conceive, be satisfactorily made in any case. When the sextant is held in the hand, it should not be grasped tightly, as this causes tremor. The handle should be fitted to the observer's hand. There is scarcely ever hold enough given. When observing an altitude at sea, there is a little difficulty in bringing down the object to the point of the horizon immediately under it. But as the shortest distance is the true distance, by running along the horizon and keeping the object in the field, the direction in which the object should be observed is easily found, and the contact made there. In observing lunar distances the great art is to turn the instrument round the line of sight exactly as if the telescope were an axis of rotation. The index is set to the approximate distance, and the fainter object viewed directly ; when the plane of the instrument passes through both objects, the brighter will come into the field, and the contact is to be made by the tangent-screw, or nearly so. We think it is better in all cases where the angle between the objects is increasing or decreasing, to make the contact open or close, and then try to seize on the moment when the contact is perfect, or the two moments when the contact begins and when you conceive it to end. The mean will probably be near the truth. All contacts must be made scrupulously in the centre between the two parallel wires. When the angle is large,
inattention to this point will cause considerable error.
On shipboard, the observations, except those of lunar distance, are necessarily rude and imperfect, the sea horizon is generally ill defined and badly seen, the dip is somewhat uncertain, and as the single altitude is observed, all the error committed tells upon the final angle. The accuracy of observation is however equal to the wants of naviga tion in ordinary circumstances. (A dip aector might be useful where more than usual accuracy is required.) On land, where the altitudes are taken from a mercurial horizon, and the sextant fixed on a stand, the observation is capable of great accuracy, and does not skill or delicacy in mere handling. There is one peculiar advantage in sextant observations, that when the mercury is quiet enough to reflect a well-defined image, there is no need of any further steadiness or of a second observer, one of which conditions is absolutely necessary in every other instrument in common use which is adjusted by a plumb line or level. We think the capabilities of the sextant as a geogra phical instrument have not been generally appreciated by English travellers; certain it is that few have used it with all the!' profit they might have done.
The mercurial horizon is a shallow rectangular wooden or iron trough, into which a small quantity of mercury is poured. If there is any wind, the trough must be covered with a penthouse, the sloping sides of which are glazed with plates of glass ground very flat and true. Any object seen in the mercury appears to be just as much below the horizontal plane as it is really above it ; hence if the angle between any heavenly body and its reflection be measured, this tingle is just twice the actual altitude of the body at that time above the horizon of the place. Suppose the altitude of the nun is to be observed in order to determine the time : The trough is placed with its largest dimen sion towards the sun, and the roof so that the sides cast no shadow, the proper dark glass is affixed to the telescope, the images brought to equal brightness, the index error ascertained, and the telescope is then directed to the image of the sun in the mercury. Holding the plane of the sextant vertical, and moving the index forward gradually, the image of the nun reflected by the index-ease will appear to enter the field from below. If the sun be rising, the index should be carried forward until the two images, having crossed, are clear of each other, and then the index is clamped. The two images will gradually close, and the exact moment by the chronometer is to be noted when the contact takes place. If the sun is rising with tolerable rapidity, the contact of the upper limb, that is, the separation of the two images, should be noted, exactly as before, without touching the clamp. The index is then read off; it may then be put forwards, and the observa tion repeated. If the interval between the contacts of the lower and upper limb is found to be in both eases nearly the same, the observer may be satisfied that be has committed no great error in noting the momenta of contact. Where the"greatest accuracy is required, it is usual to set the index to a whole number of degrees for the first pair of observations, and to put It forward 1' 30' or 2* for the second pair, and so on till the observer is satisfied or wearied ; and then in the afternoon to set to the highest reading for the first pair, and so on backwards till you arrive at the first reading. Collecting the corresponding pairs, you have an approximate time for apparent noon, which, after being corrected by the equation to equal altitudes for the change of the sun's declination, gives the time of apparent noon by the chronometer ; and this, when corrected a second time by the equation el time taken and interpolated from the Nautical Almanac,' gives the time of mean noon by the chronometer, and consequently the error of the chronometer on mean time at the place. If the altitudes are very low, and the barometer and thermometer have changed considerably between the observations, a corresponding correction must be applied, bilt in most cases this is not requisite. The time may in this way be generally got as accurately as the chronometer will keep it during the interval. When observations of the sun are made for the latitude, the altitudes should be taken-continuously for several minutes before and after apparent noon, observing the upper and lower limb alternately, or at least an equal number of each : thus getting rid of the sun's semidiameter, besides bringing more divisions into play. It is not easy to give a definite rule for the extent of these observations on each side noon ; but when the time is known, and the altitude of the sun not great, we have never scrupled to go as far as the Tables of Reduction, that is, a good half hour on each side of the meridian. It is of great importance however, when the observations are pushed so far, that4the number of observations on each side, and the times at which they are taken, should nearly correspond. If this precaution be observed, a slight error in the actual time of noon does not matter. 'When time is determined by absolute altitudes, the roof of the mercurial horizon should be reversed after half the observations, and the same precaution should be adopted when observations are made for the latitude. For equal altitudes it is necessary to keep the same position of the roof in both cases.