2 



EXPLANATION OF SCIENTIFIC TERMS. 



used in place of the weight or the pres- 

 sure of the atmosphere. See Atmos- 

 phere. 



AIR, RAREFIED. See Rarefaction. 



AIR-TIGHT, that degree of closeness in 

 any vessel or tube which prevents the 

 passage of air. 



AIR-VESSEL, a vessel in which air is 

 condensed by pressure, for the purpose of 

 employing the re-action of its elasticity 

 as a moving power. 



AMETHYST. See Corundum. 



AN A LCI ME is a stone which is found 

 " in grouped crystals, deposited by water, 

 in the fissures of hard lavas." It melts 

 under the blowpipe into a semi-transpa- 

 rent glass. It is also called Cubizite.- 

 See Polarisation of Light, page 39. 



ANGLE. When two straight lines, not 

 lying in the same direction, as A C and 

 B C, meet in a point, as at C, the open- 

 ing between them is, in common lan- 

 guage, called a nook, or corner ; and, in 

 Geometry, an angle. Thus, the opening 

 at C is called the angle A C B. 



Mathematicians have modes of ex- 

 pressing the comparative extent of such 

 openings, or angles. Thus, in figure 1, 

 draw, round C, as a centre, a circle b n a 

 dgef, extending the line BC until it 

 meet the circle, which will be thus cut 

 into two equal parts, or Semicircles. Let 

 the circumference of this circle be di- 

 vided into 360 equal parts (for all circles 

 are supposed to be so divided) and the 

 number of those parts that are contained 

 in the portion anb, which is called an 

 Arc, is the measure of the angle A C B. 

 As the figure is here drawn, the number 

 of parts are forty, and, therefore, A C B 

 is said to he an angle of forty Degrees, 

 and thus marked 40. Every degree is 

 r --pposed to be subdivided into sixty equal 

 rts, called Minutes, and those again 

 to sixty still more minute parts termed 

 wonds, and even Thirds, each a sixtieth 

 .rt of a second, are calculated by astro- 

 >mers Such subdivisions, however, 

 .n refer only to circles of a large diame- 

 r, and are measured by means of in- 

 ruments. See Vernier. 

 A whole circle containing 360, the se- 



micircle will contain 180; and if, at the 

 point C, we draw a straight line C d, so as 

 to cut the semicircle into two equal parts, 

 or Quadrants, each of these quadrantal 

 arcs, dab, and dge, will contain 90, 

 being the measure of the angles d C B 

 and dCe, which, being equal, are each 

 termed a Right angle ; and the line C d, 

 neither inclining to the right hand nor 

 the left, is called a Perpendicular to the 

 line e b, the Diameter of the circle. Any 

 line from the centre C to the circum- 

 ference, as C e, C rf, C a, and C b, for they 

 are all equal,is the Radiiis. When an angle 

 is less than 90, it is called an Acute angle, 

 such as A C B first mentioned ; but when 

 it exceeds a right angle, as A C e does, it 

 is said to be Obtuse. 



If, on the same figure, we draw a line 

 bh, perpendicular to C B, touching the 

 circle at b and the line A C at h ; then 

 h b is termed the Tangent, and AC the 

 Secant of the angle A C B ; that is, they 

 are the tangent and the secant of an angle 

 of 40, when C b is the radius. See 

 Tangent. 



Again ; if, from the point a, we draw 

 another line ai, also perpendicular to 

 C B, this line (a /) is termed the Sine of 

 the same angle A C B ; and the part i b, 

 (cut off from the semidiameter or radius 

 C b} is the Versed Sine. A straight line 

 a b drawn from a to b is called the Chord 

 of the arc amb. It is the cord or string 

 of the bow (the Latin arcus). These 

 lines are the sine, tangent, &c. of the 

 arc or angle of the circle here represented ; 

 but were it increased ever so much, the 

 number of degrees would still be the 

 same, though larger, and the lengths of 

 the sine, tangent, &c., would bear the 

 same proportion to the new radius as they 

 now do to C B. 



ANGLES of INCIDENCE, REFLEX- 

 ION, and REFRACTION. See Reflex- 

 ion and Refractive Power. 



of DRAUGHT. When a power 



is applied to drag or roll a body over a 

 plane surface, it has to overcome two ob- 

 stacles : one is the friction of the surface 

 over which the body slides or rolls ; and 

 the other is the weight of the body itself. 

 There is, in every case, a certain direc- 

 tion of the drawing power which is best 

 adapted to overcome these conjoined ob- 

 stacles ; and the angle made by the line 

 of direction with a line upon the plane 

 over which the body is drawn, and per- 

 pendicular to that line of direction, is 

 termed the Angle of Draught. Calcu- 

 lations on this subject may be seen at 

 pp. 19 26 of Mechanics, Treatise iii. 



ANHYDROUS. See Hydrate. 



APEX. See Cone. 



APOpHYLLITE,orFISH-EYE-STONE, 

 is a scarce mineral, having a pearly lustre, 

 like to the species of feldspar called moon- 

 stone. Its crystals are various, and often 



