433 



SECTOR. 



SEDITION. 



passes which can be opened or shut at pleasure. Suppose two scales' 

 say of chords, to be laid down on o A and o B, which are chords of 90) 



o P and O Q being radii, or chords of 60. If o P be four inches, we 

 have then before us two scales of chords with the radius of four inches 

 actually laid down, and any chords might be taken off them as from a 

 common scale : for instance, if the marks of 35 be at c and D, then 

 either o c or o D is the chord of 35 to a radius of four inches. But 

 suppose it required to find the chord of 35, not to a radius of four 

 inches, but to one of 3'61 inches. We know that 



4 : 3-61 : : ch. 35 (rad. 4) : ch. 35 (rad. 3'61) ; 



and the fourth term of this proportion is to be found. Now this may 

 be done with sufficient accuracy, and without any drawing, as follows : 

 Take a common pair of compasses, and open them to 3'61 on a sub- 

 divided scale of inches. Then open the sector until the points of the 

 compasses are made to fall on V and Q, which, if the sector open 

 rather easily, may be done very quickly. We have then PQ = 3'61 

 inches, and c D is the chord of 35 to that radius ; for by similar 

 triangles 



OP : PQ : : oc : CD; 



or, 4 : 3-61 : : ch. 35 (rad. 4) : c D ; 



whence, CD = ch. 35 (rad. 3-61). 



Take the compasses then, and fixing one point at c, make the other 

 fall on D ; the distance c D may then be transferred with the compasses 

 to the paper, or to the scales of inches, according as construction or 

 arithmetical estimation is required. 



The scales usually laid down upon the two sides of common sectors, 

 such as are constructed upon a foot ruler, are : 



1. A line of pnlygms, marked POL., showing the sides of polygons 

 inscribed in a circle. And since the side of the hexagon is the radius 

 of the circle, the radius of the scale is the distance from o to the 

 figure 6. Thus, t<> inscribe a polygon of ten sides in a circle of two 

 inches radius, open the sector until 6 and 6 on the counterpart poly- 

 gon scales are two inches apart : then the distance from 10 to 10 on the 

 same Kales will be the side of the figure required. 



2. A line of chord*, as above described, the radius being the chord 

 of 60. But it is to be noted that upon the common sector the whole 

 length o A of the scale is only the chord of 60, so that angles above 

 60* do not appear. To construct an angle of more than 60, first set 

 off 60* on the circle drawn by means of the radius, and take the chord 

 of the remainder from the sector. We think that the sector might 

 be usefully copied in this respect on the common scales : giving all the 

 length which the proposed scale will allow, to the chord of 60, instead 

 of that of 90. 



3. A line of rinet, the radius of which is the sine of 90. Thus, to 

 find the upright side of a right-angled triangle having an hypothenuse 

 of 3'41 inches, and an angle of 32, open the sector until 90 and 

 90 on the counterpart lines of sines are 3-41 inches apart; then 

 the distance from 32 to 32 on the same lines will be the side 

 required. 



4. A line of tecatUt, usually extending to about 75, the radius of 

 coarse being the secant of 0. This scale is blank from o to 0. 



6. A first line of tangents, from to 45*, the radius being the tan- 

 gent of 45, or the whole length of the rule. 



6. A second line of tangents, on a smaller scale, beginning at 45 

 and proceeding to about 75", the radius being the tangent of 45, or 

 the distance from o to the beginning of the scale. 



7. A line of equal parts for operations answering to finding a fourth 

 proportional to three numbers. Thus to find a fourth proportional to 

 36, 47, and 53, make 36 and 36 on the counterpart scales fall as far 

 apart as from o to 47 ; then the distance from 53 to 53 on the same 

 soales, measured on the scale, will show the fourth proportional. 



To write on the applications of the sector would be to make a 

 treatise on graphical trigonometry : one instance may suffice. It is 

 required to calculate the formula 



sin. 69 

 76 x 



On the counterpart lines of sines make 38 3 and 38 (by opening the 

 Hector) fall as far apart as from o to 59 on the same scale ; then the 

 distance from 76 to 76 on the counterpart lines of equal parts will 

 show, on that line of equal part*, the numerical value of the result 

 required. Or make 38 and 38 on the counterpart lines of sines fall 

 as far apart as from o to 76 on the line of equal partH : then the dis- 



ABTS AM) 8CI. DIV. VOL. VII. 



tance from 59 to 59 on the counterpart lines of sines will show tha 

 result required on the scale of equal parts. 



The sector becomes an incorrect instrument, comparatively, when d 

 great opening is required, and also when the result is much greater 

 than the data from which it is produced. So much accuracy of con- 

 struction is necessary, that those to whom the instrument is often 

 really useful (not many, we suspect) should rather procure the larger 

 ones, which are manufactured by the best instrument-makers, than be 

 content with the six-inch lines which are found in the common cases 

 of instruments. The sector is an instrument which requires much 

 more care than the common scale, and in the use of which expertnesa 

 can be gained by nothing but practice. Each scale is a pair of parallel 

 lines with cross divisions ; and it is important to note that the com' 

 passes must be applied to the inner of the parallel lines in every case. 

 Also when the compasses are in the hand, with one point laid on one 

 of the scales, the other scale being about to be moved to bring the 

 other point of the compass on the right division of the counterpart 

 scale, take care to hold the compasses only by that leg which is laid 

 down on the scale. 



SECTOR, ZENITH. [ZENITH SECTOR.] 



SECULAR, a name given to those variations in the planetary 

 motions which are of long duration, so that their periods are better 

 expressed in centuries than years. 



SEDATIVES are agents which produce a direct depression of the 

 action of the vascular system, with little sensible evacuation. They 

 differ from narcotics, inasmuch as their depressing effects are not pre- 

 ceded by any obvious excitement or increased action of the heart and 

 arteries. Whether they act primarily on the heart itself, or secondarily 

 by a previous influence on the nervous system, is not clearly ascer- 

 tained. Some, such as the infusion of tobacco, and hydrocyanic acid, 

 appear to destroy completely the sensibility of the heart, so that it no 

 longer responds to the stimulus of the blood; but how this effect 

 results is altogether unknown. Oxalic acid, when the dose is large, 

 seems also to paralyse the heart ; while in less quantities it operates 

 differently. [OXALIC Aero.] The peculiar mode of action of the 

 articles entitled to be considered as pure sedatives has been detailed 

 under the respective heads of DIGITALIS, HYDROCYANIC ACID, Nico- 

 TIANA, kc., and need not be repeated here. The medical employment 

 of these formidable agents should never be had recourse to without 

 competent authority and superintendence ; but as many cases of 

 poisoning result from their accidental or criminal administration, it is 

 needful to observe that the greatest promptness is requisite in the 

 administration of appropriate remedies. Vital stimulants, such as 

 ammonia and brandy, are the best; and electricity or galvanism may 

 be resorted to after the others. 



Sulphuretted hydrogen, when breathed, injected into a vein of the 

 rectum, or even applied to the skin, acts as a sedative, and in a small 

 quantity can occasion death. It is largely disengaged from many 

 decomposing substances, such as exist in stagnant pools, ditches, 

 drains, and cesspools. Proximity to these produce effects more or less 

 serious according to the intensity of the gas. Even one of its com- 

 pounds, hydro-sulphuret of ammonia, is a potent sedative. Cold, 

 when extreme, likewise acts as a sedative, but its mode of action has 

 been already explained under BATHING. Chloroform and other anaes- 

 thetics may be considered sedatives of the nerves of sensation. 



SEDILIA, in ecclesiastical architecture, the seats on the south side 

 of the chancel, near the high altar, in Roman Catholic churches, in 

 which the officiating priest and his attendant ministers sit during a 

 portion of the performance of high mass. In many of our old collegiate 

 and parish churches the sedilia still remain. The few that are extant 

 of Norman date belong to the middle or latter half of the 12th century. 

 In Pointed Gothic of every period they are frequent, and are almost 

 always made a very ornamental feature. Usually they are recessed in 

 the wall, like niches, and covered with canopies, which in some 

 instances are lofty and much ornamented, resembling tabernacle work : 

 but sometimes the sedilia are quite plain, little mure in fact than stone 

 benches. Commonly the sedilia are three in number, divided from each 

 other by slender pillars, and gradated, that nearest the altar (intended 

 for the priest) being the highest ; but they are met with of varying 

 numbers from one to five and all level, or with only the priest's seat 

 raised above the others. 



SEDITION (from the Latin teditio). It is stated that in many 

 of the old English common law writers treason is sometimes expressed 

 by the term Sedition ; and that when law proceedings were in Latin, 

 seditio was the technical word used in indictments for treason, till it 

 was superseded by the word prodUio. 



Sedition does not appear to be very exactly defined. It is stated to 

 comprehend contemptuous, indecent, or malicious observations upon 

 the crown or government, whether made in words only, or in writing, 

 or by tokens (which last term must comprehend pictures or drawings), 

 calculated to lower the sovereign in the opinion of the subjects or to 

 weaken his government. All these offences fall short of treason ; but 

 they are considered crimes at common law, and punishable by fine and 

 imprisonment. 



There are also statutes against particular acts of sedition, such as 

 seditious libels. [LAW, CRIMINAL.] There are also various acts against 

 societies established for seditious and treasonable purposes, and against 

 seditious meetings and assepoblieg. 



