SEXTANT 



SEXTON 



353 



ACB is the angle between the original and finally 

 reflected rays, and ADB is the angle between the 

 mirrors. Now, as the angle of reflection is equal 

 to the angle of incidence, L SAF = L BAD, and 

 L GBA = L DEC ; but L EBC = L BAC + L BCA 

 = ( L BAD + L DAC) + L BCA = ( L BAD + 

 L SAF) + L BCA = 2 L BAD + L BCA ; and L EBC 

 also = L EBD + i DBC = L EBD + L GBA = 

 2 L EBD = 2 i BAD + 2 ^ BDA ; therefore 

 / BCA = 2 i BDA, which proves the truth of the 

 theorem. The instrument of which this theorem 

 is the principle is a 

 brass sector of a circle 

 in outline ; the sector 

 being the sixth part 

 of a complete circle, 

 for which reason the 

 instrument is called 

 a sextant. Fig. 2 

 shows the essentials 

 of its construction ; 

 "C AMN is the sector 

 whose curved side, 

 MIN, is the sixth 

 part of a circle ; A 

 is one mirror wholly 

 silvered, placed per- 

 pendicular to the 

 plane of the sector, 

 and on, and in line 

 with, the limb AI, 

 which is movable 

 round a joint at or 

 near A ; B is the 

 other mirror, also perpendicular to the plane of the 

 instrument, and silvered on the lower half only, 

 the upper half being transparent ; E is an eyelet- 

 hole or small telescope. The graduation runs from 

 N to M (on a slip of silver, platinum, or gold let 

 into the rim ), and is HO adjusted that, when the 

 movable limb is drawn towards N till the mirrors 

 A and B are parallel, the index which is carried 

 at the foot of the movable limb is opposite zero 

 on the graduation. If we suppose that this zero- 

 point U at N, it is evident that the angle between 

 the mirrors is equal to the angle NAI ; and again, 

 if instead of graduating from at N to 60 at M, 

 which is the proper graduation for the sixth part 



Kg.L 



Fig. 2. 



of a circle, the graduation be made from to 120 

 i.e. each half degree being marked as a degree, 

 and similarly of its aliquot parts then the angle 

 NAI, read off by the index at I, will show at once 

 the angle between the incident and finally reflected 

 rays. The mode of using the sextant consists in 

 placing the eye to the telescope or eyelet-hole, and 

 observing one object directly through the unsilvered 



part of B, and then moving the index till the image 

 of the other object, reflected from A upon the 

 silvered part of B, coincides with or is opposite to 

 the first object ; then the angle, read off at I, gives 

 the angle between the objects. For additional 

 accuracy a vernier is attached to the foot of the 

 movable limb. 



The sextant is capable of very general application, 

 but its chief use is on board ship to observe the 

 altitude of the sun, the lunar distances, &c., in 

 order to determine the latitude and longitude. For 

 this purpose it is necessary to have stained glasses 

 interposed between the mirrors A and B, to reduce 

 the sun's brightness. These glasses (generally 

 three in number) are hinged on the side AM, so 

 that they may be interposed or not at pleasure. 

 B is the glass through which the horizon is per- 

 ceived, and has hence received the name of the 

 horizon-glass; while the other mirror, from its 

 being attached to the index-limb, is called the 

 index-glass. 



The sextant is liable to three chief errors of 

 adjustment : 1 if the index -glass be not perpen- 

 dicular to the plane of the instrument ; 2 if the 

 horizon-glass be not perpendicular to the plane of 

 the instrument ; ana 3 if, when the mirrors are 

 parallel (which is the case when a very distant 

 body, such as the sun or moon, is observed directly 

 through B, and found to coincide with its image 

 in the lower part of B), the index does not point 

 accurately to ; this last is called the index-error, 

 and is either allowed for, or is remedied by means 

 of a screw, which moves the index in the limb AI, 

 the latter being stationary. The first two errors 

 are also frequently remedied by means of screws 

 working against a spring, but in the best instru- 

 ments tin- maker himself fixes the glasses in their 

 proper position. The quadrant differs from the 

 sextant only in having it* arc the fourth part of a 

 circle, and being consequently graduated from 0" 

 to 180 ; the octant contains 45, and is graduated 

 from to 90 ; while the repeating-circle, which is 

 a complete circle, is graduated from to 720. A 

 common form of the sextant is the ' snuff-box ' 

 sextant, which is circular in shape, and, as it can 

 be conveniently carried in the pocket, is the form 

 most frequently used by land-surveyors. 



The idea of a reflecting instrument, on the 

 principle of the sextant, was first given by Hooke 

 about 1666 ; but the first instrument deserving the 

 name was invented by John Hadley (q.v. ) early 

 in the summer of 1730, and a second, and much 

 improved form of it, was made by him a short 

 time afterwards. Halley, at a meeting of the 

 Royal Society, claimed for Newton the priority of 

 invention, and in October 1730 a Philadelphia!), 

 named Godfrey, also asserted his claim as the 

 original inventor ; but that learned body decided 

 that Newton's claim was unsupported by even 

 probable evidence, and that Hadley'sand Godfrey's 

 inventions were both original, but that the second 

 form ( which is almost the same as the common 

 sextant now employed) of Hadley's instrument 

 was far superior to his first form and to Godfrey's. 

 See works by H. W. Clarke (1886) and C. W. 

 Thompson (1887). 



Sexton (corrupted from Sacristan, q.v.) is a 

 parochial officer in England, whose duty is to take 

 care of the things belonging to divine worship. He 

 is usually chosen by the inhabitants, but often also 

 by the clergyman or the churchwardens, the mode 

 of appointment being regulated by the custom of 

 each parish. He sometimes also holds the office 

 of parish-clerk. Women have occasionally been 

 appointed sextons (e.g. at Kingston-upon-Thames, 

 1731 ; Doanybrook, near Dublin, 1845-56 ; and the 

 adjoining parish of Booterstown, 1856-74). The 

 ofhce is a freehold office for life, except in the 



