20 



EXPLANATION OF SCIENTIFIC TERMS. 



ffc by two-fifths; Jc d by three-fifths; 

 m e by four-fifths ; and I S by five-fifths, 

 or a whole division. By this means each 



Fig. 14. 



of the divisions, A B, B C, &c. may be 

 divided into five parts, or minutes, by 

 shifting the moveable limb (or vernier} 

 H I, to any part of the quadrant where 

 the subdivision is required. This appen- 

 dage is also added to right lined scales, as 

 may be seen commonly on barometers, 

 by which the inch is first divided into 

 tenths, and a sliding vernier applied so 

 as to subdivide the inch into hundredths. 

 The vernier was invented about the year 

 1630, and was long termed a Nonius ; 

 because it was considered as merely an 

 improvement of the method of subdivi- 



! sioii adopted by Nunnez, a Portuguese, 

 \vho lived about a century previous. The 

 method of Nunnez was, however, diffe- 

 rent. He drew upon the quadrant a num- 

 ber of circles (46) concentric with the 

 limb, each of which he divided diffe- 

 rently : thus, the limb had 90 parts ; 

 the first circle 89 ; the second 88, &c. 

 When he made his observation he marked 

 the division on which it fell, (it mattered 

 not on what circle,) and proportioned this 

 division to that of 90 on the limb. Thus, 

 if it cut the twenty-third division of 

 the quadrant which was divided into 

 82 parts, he said, if quarter of a circle di- 

 vided into 82 parts has given 23 divisions, 

 how many would it have given in a qua- 

 drant of 90 parts ? and by this calcula- 

 tion he satisfied himself that the real 

 angle was 25 degrees, 14 minutes, and 

 38 seconds, the exactitude of which must 

 have depended much upon the size and 

 the accuracy of the construction of his 

 instrument. 



VERNIER, CHROMATIC, is an instru- 

 ment so called and invented by Dr. Brew- 

 ster, for the purpose of measuring, by 

 comparison, the very minute variations 

 of tints. We must refer to page 53 of the 

 treatise on Polarisation of Light for the 

 description of this vernier, as we find it 



impossible to give a simpler explana- 

 tion. 



VERSED SINE. See Angle. 



VERTEX. See Cone, and Conic Sections. 



VESUVIAN, or IDOCRASE, is a stone 

 generally of a reddish-brown colour, simi- 

 lar in appearance to common garnet. It 

 is found, crystallized, among substances 

 thrown out by volcanoes ; and, as its 

 name indicates, particularly by Mount 

 Vesuvius. 



VIS INERTLE. According to Newton, 

 every body perseveres in the same state, 

 either of rest or of uniform motion, in a 

 right line, unless it be forced to change 

 that state by a foreign force. This inert- 

 ness, or principle of inactivity, is called 

 by the Latin name Vis inertice. 



VOLUME. The apparent space which a 

 body occupies is termed its volume ; the 

 effective space which the same body occu- 

 pies, or its real bulk of matter, is its mass? 

 the relation of the mass to the volume (or 

 the quotient of the one by the other) is- 

 its density ; and the empty spaces or voids, 

 which render the volume larger than the 

 mass, are its pores. 



WATER-TIGHT is that degree of close- 

 ness in a vessel, or tube, which prevents, 

 the passage of water. 



WEIGHT is the comparative measure of 

 the gravity of bodies at the earth's sur- 

 face. 



OF THE ATMOSPHERE. 



See Atmospheric Pressure. 



WIND is air in perceptible motion. 



ZERO (nothing) is with us applied only as 

 a scientific term ; but by the French,, 

 generally, to denote a cypher (0) ; while, 

 in that language, un chiffre denotes any 

 of the digits, or arithmetical figures. Both 

 words appear to be of similar origin; 

 and probably from the Arabic tsaphara, 

 empty or void. In this literal sense, it 

 fills the blank between the ascending and 

 descending numbers in a series. See 

 Scale. In common language, to cypher 

 is to calculate; and to write in cyphers is 

 to write in secret or unknown characters, 

 such as were the Arabic numerals wheu< 

 first introduced into Europe. 



ZIRCONE is a heavy, hard, sparkling, 

 and transparent stone, susceptible of a 

 fine polish, and having a strong double 

 refraction. It is usually divided into 

 the two varieties of Hyacinth and Jargon; 

 the former having a yellowish -red colour^ 

 and the latter being most esteemed when- 

 colourless. 



