493 



SEXTANT. 



SEXTANT. 



4P4, 



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 ia better in all 

 cases where the augle 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 urctor 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 require much 

 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 u really above it ; hence if the angle between 

 any heavenly body and its reflection be measured, this angle is just 

 twice the actual altitude of the body at that time above the horizon of 

 the place. Suppose the altitude of the sun 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 sun reflected by the index-glass 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, 



!i uld 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 

 tipper limb is found to be in both cases nearly the same, the observer 

 may be satisfied that lie has committed no great error in noting the 

 iji.'in. nts 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 iff 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 

 kackxardt 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 tquatian to ttpial 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 

 i 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 

 Itetween the observations, a corresponding correction must be applied, 

 I <*i t in most cases this is not requisite. The time may hi this way be 

 generally got as accurately aa 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 Seduction, 

 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, that-the 

 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. 



In the observations already described, and for lunar distances, a 

 stand for the sextant will indeed add greatly to the accuracy of the 

 observations, but is not absolutely necessary for a practised observer ; 

 so far as our own experience has gone, good observations of the stars 

 can scarcely be made without a stand. By lying on the ground, and 

 resting the edge of the sextant on a book or some convenient support, 

 the observation may undoubtedly be made; but in most situations 

 and in many climates this would expose the observer to more incon- 

 venience and risk than he can prudently encounter. With a stand, 

 and a little experience in setting it up, the observation of a star is just 

 as accurate as that of the sun. The time is noted when the images 

 form a figure like this **, parallel to the horizon ; and stars of the 

 third magnitude, or even lower, are quite bright enough for the pur- 

 pose. We have observed Polaris very tolerably with a snuff-box sex- 

 tant when the instrument rested on a book ; the telescope, though of 

 extra size for the instrument, was of much smaller aperture than those 

 attached to ordinary sextants. It is rather steadiness which is required 

 for these observations than light. Equal altitudes of the same star 

 give the moment of its meridian passage by the chronometer without 

 computation ; and as this is the R.A. of the star, the error of the chro- 

 nometer, if it be sidereal, is obtained at once ; if it should be a solar 

 chronometer, the computation is very easy, that is, merely computing 

 the mean solar time of the star's transit of that day from the K.A. of 

 the star and the longitude of the place. The computation of rircum- 

 meridian altitudes of a star for the latitude is rendered more simple by 

 using it sidereal chronometer, for the hour angles observed with the 

 solar chronometer must be reduced to sidereal time before taking out 

 the corrections to the meridian from the Tables of Reduction. 



There are certain faults either usual or inevitable in the sextant, 

 which are now to be pointed out, and the mode of eliminating their effect 

 from the final result. First, the determination of index correction is 

 always somewhat erroneous, and this error runs through all the abso- 

 lute observations, and affects the mean with its whole amount. Again, 

 if the telescope be not parallel to the plane of the instrument, or if 

 the contact be not made exactly midway between the wires, the angle 

 read off is too large, and exceeds the true angle, and there is no prin- 

 ciple of compensation ; this class of errors has always the same sign. 

 Finally, as sextants are now divided, there is no certainty that the 

 axis of the index is exactly concentric with the divided arc. It is 

 difficult to say within what limits this error is confined, but we think 

 it amounts not unfrequently to 10", 15", or even 20" in the total arc.* 

 This is not, however, proportional to the arc, though it is generally 

 smaller in the smaller arcs. Hence if this effect of eccentricity lie the 

 same way as the other errors, as it easily may, the most careful ob- 

 server will make a considerable mistake in measuring a large angle. 

 If we further suppose the error of the lunar tables to be in the same 

 direction (the best meridian observations show that the moon is not un- 

 frequently from 10" to 15" out of her tabular place), it will be seen that 

 the longitude, determined as it generally is by distances between the 

 moon and sun, or the moon and a ringlr. star, may differ much more 

 from the truth than lunar observers usually think possible, f 



Mere instrumental error is got rid of by combining those observa- 

 tions which they affect in contrary ways. Thus in equal altitudes, the 

 Jued errors, such as index error, excentricity, bad division, inclination 

 of the glasses or telescope, have contrary effects on the morning and 

 afternoon sights, and therefore no effect on the mean. Making the 

 contacts out of the middle of the wires is a casual error, and just as 

 likely to happen in the morning as in the evening observations ; so 

 that as the angles are the same in both cases, errors of this kind will 

 have pretty much the same influence on the mean of several pairs. 



* Mr. Simms i of opinion that with a dividing engine of a better construction, 

 this error of excentricity may be reduced to 2 " or 3". 



f We think no prudent seaman would rely upon lunar distances at sea to a 

 smaller quantity than one-third of a degree. At land a good observer should 

 seldom be out more than an eighth of a degree ; and a large mass of careful 

 observations, corrected for the errors of the lunar tables, might perhaps come 

 within the sixteenth or twentieth of a degree. This seems to us almost the 

 limit of what is to be expected from lunar distances, and it is inferior in accu- 

 racy to one favourable lunar transit, with a tolerable telescope, a well practised 

 observer, and a corresponding observation at a principal observatory. 



