1119 



NAVIGATION NAUTICAL ASTRONOMY. [QUADRAHT AND n\ 



degree* beyond 00. With cither of thcso instrument* 

 an altitude may bo taken ; l>ut usually Klf. tt. 



with more precision with the sextant 

 than with the quadrant, on account of 

 the more minute uUI:\iv..H< of the 

 nrc. The manner of holding the in- 

 Mruinrnt in taking an altitude is 

 figured in the margin. (Seo Fig. 32). 



The sextant, however, is almost ex- 

 clusively used for taking a 1m 



'All the essential parts of this 

 valuable instrument are represented in tho accompanying 

 figui. 33) is tho handle by wliich th.e sextant 



Fig. J3. 



fig. SI. 



is held while taking the angular distance ; D E is a small 

 telescope fixed to the frame of tho instrument, and 

 directed towards tho piano reflector or mirror, C. This re- 

 flector is immovable ; the other plane reflector or speculum. 

 15, is fixed to the movable radius, or movable limb, F, 

 at the extremity of which is the index for pointing out 

 the angle measured on the graduated limb, A A. 



Tin- two reflectors, B 0, are perpendicular to the plane 

 of the instrument that is, to the plane in which tho 

 arc, or graduated limb, lies. The immovable reflector, 

 C, is called the horizon glass ; the upper half of it is trans- 

 parent and unsilvered ; and it is through this part that 

 the horizon is viewed in taking an altitude ; the lower 

 half only, strictly speaking, is a reflector, being the only 

 part coated with quicksilver (Fig. 34). The movable 

 reflector, B, is called the index glass ; it 

 turns with the radius, or limb carrying the 

 index, round the centre of the graduated 







Suppose this radius to bo turned into such 

 a position that the planes of the two re- KS^ 

 Hectors, B, C, may be exactly parallel to one anot 

 this position of the index limb, the point on the 

 ated arc shown by the jndex is to be marked 0. It is 

 the perfection of tho instrument that, when the index 

 point* to 0, tho two reflectors should be accurately 

 parallel to one ;l n ,ther. And here wo pause for a 

 moment to explain the cause of what the learner may 

 hare hitherto considered as tho result of indifferent 

 workmanship in the its employed in the pre- 



ceding problems of Nautical Astronomy. We have 

 repeatedly spoken of tho index error, or the error of tfo 

 instrument. 



These terms do not imply faulty workmanship ; all 

 instruments whatever tint most elaborately-finished 

 specimens in th most richly-furnished observatories 

 have, without exception, their inxtniinvntal errori, win. U 

 are very different things, however, from errors of work- 

 iiip. 



I n ih'i wet ion on Mathematics (Commentary on En el id), 

 reader ha* bee that it is 



beyond the roach of art to draw a lino nf ft preHc: 

 length fuxnrat'ly ; to raise a perpendicular ; or to draw a 



r : in 



radii- 



pair of lines that shall bo accurately parallel. The. per- 

 fect periH'ndiciilanty and parallelism, therefore, of thu 

 reflectors, B, C, are thing* that cannot be practically 

 brought about ; some minute departure from strict 

 geometrical precision always exists, and hence, what is 

 called "the index error." How this index error maybe 

 discovered and allowed for, will be seen presently ; it is 

 always furnished to the purchaser by the maker of the 

 Mat 



In reference to this subject, it may not bo unprot', 1 

 to the learner, to add to the present digression the fol- 

 lowing observations of Sir John Herschel : 



" Astronomical instrument-making may bo justly re- 

 garded as the most refined of tho mechanical arts, and 

 that in which the nearest approach to geometrical pre- 

 cision is required, and has been attar 



"It may bo thought an easy thing, by one unac- 

 quainted with the niceties required, to turn a circle in 

 metal, to divide its circumference into 300 equal parts, 

 and these again into smaller subdivisions to place it 

 accurately on its centre, and to adjust it in a given 

 position ; but, practically, it is found to be one of the 

 most difficult. Nor will this appear extraordinary when 

 it is considered that, owing to the application of tele- 

 scopes to the purposes of angular measurement, every 

 imperfection of structure or division becomes magnified 

 by the whole optical power of that instrument, and that 

 thus, not only direct errors of workmanship, arising from 

 unsteadiness of hand or imperfection of tools, but those 

 inaccuracies which originate in far more uncontrollable 

 causes, such as the unequal expansion and contraction of 

 metallic masses, by a change of temperature, and their 

 unavoidable flexure or bending by their own weight, 

 become perceptible and measuralile. 



"An angle of one minute occupies, on the circum- 

 ference of a circle of ten inches in radius, only about 

 jrbv'h part of an inch a quantity too small to be ccr- 

 i dealt with without the use of magnifying glasses ; 

 yot one minute is a gross quantity in the astronomical 

 measurement of an angle. With tho instruments now 

 employed in observatories, a single second, or the COth 

 part of a minute, is rendered a distinctly visible ami 

 appreciable quantity. Now the arc of a circle, subtended 

 by one second, is less than the 200,000th part of tho 

 radius ; so that on a circle of six feet in diameter, it 

 would occupy no greater linear extent than j-'oii t; ' 1 l l:irt 

 of an inch a quantity requiring a powerful microscope 

 to be ' al all. 



" Let any one figure to himself, therefore, the difficulty 

 of placing on the circumference of a metallic circle of 

 such dimensions (supposing the difficulty of its construc- 

 tion surmounted), 300 marks, dots, or cognisable divi- 

 sions, which shall be true to their places within such 

 minute limits, to say nothing of the subdivision of th<> 

 degrees so marked off into minutes, and of these again 

 into seconds. Such a work has probably baffled, and 

 will probably for ever continue to bafflo, the utmost 

 stretch of human skill and industry ; nor, if executed, 

 could it endure. The ever-varying fluctuations of heat 

 and cold have a tendency to produce not merely tem- 

 porary and transient, but permanent, nncompenaated 

 changes of form in all considerable masses of those metals 

 which alone are applicable to such uses ; and their own 

 weight, however symmetrically formed, must always be 

 unequally sustained, since it is impossible to apply tho 

 sustaining power to every part separately : oven could 

 this l>e done, at all events force must be used to move 

 ;u:d to fix them, which can never bo done without pro- 

 dueing temporary, and risking permanent, change of 

 form. It is true, by dividing them on their centres, and 

 in the identical places they are destined to occupy, and 

 by a thousand ingenious and delicate contrivances, won- 

 ders have been accomplished in this department of art, 

 and a degree of perfection has been given, not, merely to 

 chrfs-tftfurrf., but to instruments of moderate prices and 

 I, and in ordinary use, which, on due consider* - 

 uiust appear very i-urprisin". lint though Wl 

 ."d to look for hands of scientific 



artists, we are not to expect miracles. The demands of 



