FOR 



FOR 



1. Forces, Composition of. An expres- 

 sion employed in Mechanics, when two 

 or more forces act on a body in the same 

 direction, or in diametrically opposite 

 directions, or at some angle. In all these 

 cases, the force which represents the 

 combined effects of all the forces is called 

 the resultant. 1. In the first case, the 

 resultant is the sum of all the forces, and 

 the line of direction is one and un- 

 changed. 2. In the second case, the 

 body will remain at rest, if the forces are 

 equal; but if they are unequal, the re- 

 sultant is the difference of the forces, and 

 the direction that of the greater force. 

 3. In the third case, the resultant is a 

 mean force in an intermediate direction. 

 Thus any number of forces may be re- 

 solved into one resulting force, the effect 

 of which is the absolute motion ; and any 

 motion may be assumed to be the result 

 of a single force, or of a combination of 

 many. This is termed the composition 

 and resolution of forces. 



2. Forces, Parallelogram of. A term 

 used as explanatory of the third case in 

 the preceding paragraph. If the lines 

 which each of two forces, acting singly, 

 would have caused a body to describe in 

 a given time, make any angle whatsoever 

 with each other, the line which the body 

 will describe in that time, when both the 

 forces act upon it at the same instant, is 

 the diagonal of the parallelogram under 

 the two first-mentioned lines. 



3. Force, Accelerated. The increased 

 force which a body exerts, in consequence 

 of the acceleration of its motion. By 

 some writers this is termed accumulated 

 force. See Acceleration. 



4. Forces, External and Internal. The 

 former are those which act upon masses 

 of matter at sensible distances, as attrac- 

 tion and repulsion. The latter, or mole- 

 cular forces, are those which act only 

 on the constituent molecules of matter, 

 and at insensible distances, as cohe- 

 sion. 



5. Force, Centrifugal and Centripetal. 

 The tendency of the planets to move 

 forward in a straight line, is called the 

 centrifugal force, and the attraction of 

 the sun, by which they are drawn down- 

 wards, or towards a centre, is called their 

 centripetal force ; and it is by these two 

 forces thit the planets are made to per- 

 form their constant revolutions around 

 the sun. 



6. Forces, Polar. Those forces which 

 are conceived to act with equal intensity 

 in opposite directions at the extremities 



141 



of the axes of molecules, or of masses of 

 matter 



FO'RCER. A solid piston, employed 

 in the construction of pumps for pro- 

 ducing a constant stream, or raising 

 water to a greater height than it can be 

 raised by atmospheric pressure. 



FORE-STAFF. An instrument used 

 at sea for taking the altitudes of the 

 celestial bodies. It is now superseaed 

 by more perfect instruments. 



FOREST MARBLE. A portion of 

 the series of the Lower Oolite Formation, 

 consisting of a coarse laminated shelly 

 oolite, interposed between beds of clay, 

 sand, and grit. 



FORFICU'LID^ {forficula, a forked 

 claw). A family of orthopterous insects, 

 named from the genus forficula, and 

 constituting, according to some writers, 

 the order Dermaptera. They comprise 

 the various species of earwig. 



FORM. The mode in which an object 

 is presented to our mind or senses : in 

 the former case it belongs to the catc 

 gories ; in the latter to figure. 



FORMATION. A group of alluvial 

 deposits, sedimentary strata, or igneous 

 rocks, referred to a common origin or 

 period. In geological treatises, one or 

 more simple minerals constitute a rock; 

 one or several rocks, united by certain 

 common characters, constitute a forma- 

 tion, or connected series ; and several 

 formations constitute a system of rocks. 



FO'RMIC ACID {formica, an ant). 

 A peculiar acid extracted from red ants, 

 and procured artificially from all vege- 

 table substances by oxidation with acids. 

 Its salts are called formiates. 



FORMI'CID^ {formica, an ant). The 

 Ant tribe ; a family of Hymenopterous 

 insects, distinguished by their subterra- 

 nean habits, and the existence among 

 them of neuters, which perform the la- 

 bours of the family. 



FORMULA (dim. of forma, a form). 

 A short general form or rule for the per- 

 formance of any operation. In Algebra, 

 the expression of a general rule for the 

 solution of a problem. 



FORMULAE, CHEMICAL. A simple 

 mode of exhibiting the composition of che- 

 mical combinations by representing the 

 substances by the initial letters of their 

 Latin names, and adding figures or lines 

 to indicate the number of equivalents. 

 Thus K denotes one equivalent of po- 

 tassium ; O^, five equivalents of oxygen ; 

 N or N, two equivalents of nitrogen. 

 For every equivalent of oxygen a dot^ for 



