When a few more fundamental points in longitude are fixed, it will not be necessary to use lunar distances, except in long voyages, or in the centre of large nod uncivilised countries. The recent improve ments in chronometers, and the reduction in their price, have greatly superseded this kind of observation already, and will continue to do so more and more.
When the arc of the instrument is limited to 90°, which is usually the case in the common wooden instrument, it is called a quadrant (and sometimes an octant, as being in form an eighth of a circle), and a second horizon-glass and sight are sometimes appended for the back observation. A sight-vane (a plate of brass pierced with a small hole) is placed below the ordinary horizon-glass, and a second horizon-glass is fixed near this vane, so that lines from the centre of the index-glass, and from the latter vane, make equal angles with its plane. The plane of this second horizon-glass should be at right angles with the index glass, when the index is at zero. This adjustment may be verified by making the sea-horizon seen directly through the second vane and unsilvcred part of the second horizon-glass, coincide with the sea horizon at the back of the observer, which is seen by two reflections from the index-glass and silvered part of the second horizon-glass. The angle between the fore and back sea-horizons is evidently equal to 180° + twice the dip of the horizon. The dip is known from the observer's height above the sea-level. Suppose it to be 4', and that the reading on the back arc is 5'; it should be twice the dip, or 8' : therefore the zero position of the index for the hack observation is at 3' on the forward arc. This may be corrected by altering the second horizon glass, if there are the means of adjustment ; that is, by making the front and back sea-horizon coincide when the reading is 8' on the back arc. By moving the index forward, the reflected sun, which is behind the observer, can be brought to touch the sea-horizon in front ; and the reading, when the index error has been corrected, is 180° the sun's distance from the front horizon. Now this latter angle is evi dently 180° + 2 x dip sun's altitude ; hence it is clear the angle read off is the sun's altitude above the horizon 2 x dip. Hence sun's
altitude dip (the nititude required) = angle read off + dip. If the index error be not adjusted, the angle must he corrected for the index error ; in the preceding instance the 3' must be added to the sun's altitude. The beck observation, however, though useful in certain circumstances, is not much used at present, and many quadrants are fitted up without the second vane and horizon-glass.
If a sextant be used in the manner we have attempted to describe, the results will be found to be very satisfactory. The attentive reader will see that the principle to be seized upon is that of always making two observations at nearly the same time, in which the errors, except the casual errors of making the contacts and of reading off, are the same, but the effects of which on the final result are different. It is not always convenient to afford the time which these balanced observa tions require; and, in fact, they are often impracticable. It will, therefore, be well worth the careful observer's while to determine the constant errors of his sextant, at different angles, where he has a good opportunity, and to use this knowledge where circumstances do not allow him to make a balanced set Very small sextants are sometimes fitted in a hos, and are called snuff-bar sextants. These admit of considerable accuracy ; but, except in particular cases, they are not carefully adjusted, &c. The principle - is, of course, the same as in the larger sextants, and they have a small telescope and dark glasses. It is desirable that the box should be wide enough to admit a finger to wipe the glasses, especially when the snuff box sextant is used, as it frequently is, for maritime surveying. The salt spray very soon clouds the glasses, and there is no light to spare. For all sea-going purposes, except taking lunar distances, and for mili tary and maritime surveying, the snuff-box sextant will be found very convenient and sufficiently accurate. If made with the utmost care, and skilfully used, we think it capable of determining the latitude within 1', and the time to 3' or 4'; but an instrument-maker could not be expected to take so much trouble, unless for a favourite customer or for an expedition of some importance.