10 



EXPLANATION OF SCIENTIFIC TERMS. 





FIRST, or PRIME MOVER, in mecha- 

 n j cs . See Machine. 



FLUIDITY (Latin fluere, to flow) is that 

 state of a substance in which its consti- 

 tuent particles are indefinitely small ; and 

 so slightly cohesive, that they are move- 

 able in every direction, passing over one 

 another with the least impulse. There 

 is a partial fluidity, in which the particles 

 are condensed or thickened into a coherent 

 though tremulous mass. Jellies are of 

 this kind, and may be considered as hold- 

 ing a middle place between liquids and 

 solids. 



FLUIDS are substances, or rather masses 

 of particles, which have the quality of 

 fluidity; and, in consequence, have no 

 fixed shape, but assume that of the vessel 

 by which they are contained. They are 

 usually divided into two kinds ; gaseous 

 and liquid. 



ELASTIC. See Gas. 



NON-ELASTIC. See Liquid. 



FLY-WHEEL is an addition to certain 

 machines, for the purpose of equalizing 

 the effect of the moving power. If this 

 power act irregularly, there will be mo- 

 ments in which it will exert more force 

 than is required. This excess is employed 

 in giving motion to the fly-wheel, and 

 part of this communicated excess is re- 

 turned upon the machine when the power 

 is too languid. In the former case it is a 

 retarding, and in the latter an impelling 

 power. 



FOCUS. The Latin focus is a hearth or 

 fire-place ; and hence the word has been 

 employed to denote any point in which 

 light, and consequently heat, is concen- 

 trated. In optics, it is the point where 

 several rays are collected, whether in 

 consequence of refraction or of reflexion. 

 For the situation of the/oc of the ellipsis, 

 parabola, and hyperbola, see Conic Sec- 

 tions. 



FORCE is the name of any exertion which, 



if applied to a body, has a tendency to 



move that body when at rest ; or to 



affect, or to stop its progress, if already in 



motion. This is sometimes termed Active 



force, in contradistinction to what merely 



resists or retards the motion of another, 



but is itself, apparently, inactive. The 



degree of resistance to any motion may be 



measured by the active force required to 



overcome that resistance, and hence wri- 



mechanics make use of the terms 



ig forces and Retarding forces. 



CENTRIFUGAL. See Centri- 



LINE OF DIRECTION OF, 



traight line in which any force 

 i make a body move. 

 , COMPOSITION OF. When 

 ;i>s act on a body in the same line 

 'ion, the resulting force (or Re- 

 as it is called) will be the sum of 

 [f they act in "opposite directions, 



the body will remain at rest if the forces 

 be equal; or, if unequal, it w ^--move 

 with a force equivalent to the! differ- 

 ence, in the direction of the greater. If 

 the lines of direction make an angle with 

 each other, the resultant will be a mean 

 force in an intermediate direction. Any 

 number of forces may be thus resolved 

 into one resulting force, the effect of 

 which is the absolute motion; and any 

 motion may be assumed to be the result 

 either of a single force, or of a combina- 

 tion of many. This is what writers on 

 mechanics call the Composition and Reso- 

 lution of forces. 



FORCES, ACCELERATED. See Acce- 

 leration. 



FREEZING POINT. That point in a 

 thermometer at which the included fluid 

 stands, when the instrument is immersed 

 in another fluid that is in the act of 

 freezing, is the freezing point of the lat- 

 ter. 



FRICTION (Latin fricare, to rub) is the 

 rubbing or grating of the surfaces of bo- 

 dies upon one another. In mechanics, it 

 is considered as one of the causes of the 

 hinderance or stopping of motion, as a 

 retarding force. 



FRIGORIFIC, having the quality of pro- 

 ducing extreme cold, or of converting 

 liquids into ice : from the Latin frigus, 

 coldness. 



FULCRUM. See Lever, and Balance. 



FUSIBILITY (from the Latin fusus, 

 melted or poured out) is that quality of a 

 solid which renders it capable of being 

 brought to the state of a liquid by heat. 



FUSION is the state of melting, or soften- 

 ing into a liquid. 



GAS is an old Teutonic word, equivalent to 

 the Greek pneuma, air, or spirit, and has 

 been adopted by the modern chemists to 

 denote permanent aeriform (or airlike) 

 fluids generally, for the purpose of dis- 

 tinguishing them more clearly from com- 

 mon air, which is a mixture of two spe- 

 cies of gas. Gases are distinguished from 

 liquids by the name of Elastic fluids ; 

 while liquids are termed non-elastic, be- 

 cause they have, comparatively, no elas- 

 ticity. But the most prominent distinc- 

 tion is the following : liquids are com- 

 pressible to a certain degree, and expand 

 into their former state when the presstire 

 is removed ; and in so far, they are elas- 

 tic : but gases appear to be in a continued 

 state of compression ; for when left un- 

 confined, they expand in every direction, 

 to an extent which has not hitherto been 

 determined. Thus, a small portion of 

 common air, inclosed in a thin bladder, 

 will, when the pressure of the atmosphere 

 is removed, expand so as to inflate the 

 whole cavity, stretching out every part of 

 the surface. The expansion of a liquid, 

 under such circumstances, would not be 

 perceptible. Gases retain their elasticity 



