EXPLANATION OF SCIENTIFIC TERMS. 



wire), a continued current of Electri -ity 

 will be produced, through the conductor, 

 from one plate to the other, as loner as 

 the chemical action is exerted. This is 

 the simple Galvanic Circle. A number 

 of similar pairs of plates, placed alter- 

 nately with an acting fluid intervening 

 between each, and the two ends of the 

 series connected by a wire, multiplies the 

 effect, and is called a Galvanic, or I'ultaic, 

 pile. For the construction of such piles, 

 and of batteries of various forms, see 

 Galvanism, Chapters II. and III. The 

 end of the galvanic pile which gives out 

 the electric fluid is called thj positive 

 pole, and the other end, in which the 

 wire terminates, and which receives the 

 electric matter, is called the negative pole 

 of the pile. 



GALVANIC PILE. See Galvanic Circle. 



G A S._ See Gloss. I. 



GASEOUS signifies that the suhstance 

 spoken of has the nature of gas ; and 

 thus gaseous fluids are distinguished from 

 other fluids. 



GAS, AZOTIC. See Azote. 



CARBONIC ACID._See Carbon. 



HYDROGEN. See Oxygen. 



NITROGEN. See Azote. 



GEOMETRICAL PROGRESSION. See 

 Progression, 



GLASS DROPS. See Rupert's Drops. 



A XN E A LIN G OF. See Annealing. 



GLAUBERITE. See Gloss. I. 



GONIOMETER. See Gloss. I. 



GOVERNOR. See Gloss. I. 



GRAVITY. See Gloss. I. 



CENTRE OF. The centre 



of gravity of a solid hody is a point so 

 situated ivith respect to it, that if the 

 hody could he suspended from the point 

 in question, the whole body would remain 

 at rest (with respect to its tendency to 

 the earth) in whatever situation the 

 surrounding parts may be turned. Thus, 

 the centre of gravity of a globe, if of 

 uniform density, is its common centre ; 

 and that of a balanced beam is the pivot 

 on which it turns. See this further illus- 

 trated in the Popular Introductions, pages 

 xviii. xix. and xx. 



~- RELATIVE AND SPECI- 



FIC. The comparative or relative gra- 

 vities of different bodies towards the 

 earth are measured by a general standard 

 called weight, and one substance is said 

 to have a greater specific gravity than 

 another, when a less portion of its bulk 

 is of equal weight to that other. Thus, 

 a cubic inch of platina is nearly twice the 

 weight of a cubic inch of silver ; and, 

 therefore, is said to have double its speci- 

 fic gravity, the specific gravity of platina, 

 relative to that of silver, is as two to one. 

 In this mode of comparison, the gravities 

 are relative ; but a standard is assximed 

 with which both may be compared. That 

 standard, for solids and liquids, is water, 



which is reckoned unity, and compared 

 with that fluid, silver is 10.5 and platina 

 'J1.4 : that is, equal bulks of water, silver 

 and platina, would have weights in these 

 proportions. In designating the specific 

 gravities of gases, the standard, or unity, 

 is atmospheric air. 



GRAVITY, LINE OF DIRECTION OF, 

 is that right line which passes through 

 the centre of gravity of a body in a direc- 

 tion towards the centre of the earth. 



GYRATION (Latin gyrus, a circle) is 

 the action of turning round a centre, in 

 the manner of a wheel. 



, CENTRE OF, is a point in 



a revolving body, into which, if all its 

 matter could be collected, it would con- 

 tinue to revolve with the same energy as 

 when its parts were in their original 

 places. 



HALO, u a luminous and sometimes co- 

 loured circle, appearing occasionally 

 around the heavenly bodies, but more 

 especially about the sun and moon;" 

 See Parhelia. 



HARDNESS is the resistance to impres- 

 sion. It is incompressible, but limited to 

 solids. See Compressibility. 



HEAT. See Caloric. 



, CAPACITY FOR. See Caloric, 



Capacity for. 



CONDUCTORS OF. See Con- 

 ductors. 



LATENT. See Caloric, Latent. 



RADIATION OF. See Radiation. 



. SPECIFIC. See Caloric, Specific. 



HECATOMETRE. See Metre. 



HELIX, the scientific name for a spiral. 



HERMETIC SEAL. See Gloss. I. 



HEXAHEDRON. See Rhombus. 



HETEROGENEOUS. See Gloss. I. 



HOMOGENEOUS. See Gloss. I. 



HORIZON is a Greek word signifying a 

 boundary, and was employed to denote 

 the circle in which the apparent plane of 

 the earth terminated in the concave of 

 the sky. This is now called the sensible 

 horizon, to distinguish it from the true 

 or astronomical horizon, which is parallel 

 to the sensible, but is conceived to be a 

 plane passing through the centre of the 

 earth and dividing the whole celestial 

 sphere into the upper and lower hemi- 

 spheres. That the ancients believed the 

 earth to be an extended plain is obvious 

 from the words which they employed in its 

 description. They believed, too, that it was 

 longer from east to west than from north 

 to south : calling the former its length 

 and the latter its breadth, and hence we 

 have our terms longitude and latitude. 

 A line or plane in, or parallel to, the hori- 

 zon, is horizontal. Planes passing through 

 the zenith (or point which is directly 

 over our head) and the centre of the earth 

 are called vertical planes ; and the circles 

 in the heavens marked by such planes are 

 vertical circles, or azimuths. All these 



