FRI 



FRO 



FRICTION. The resistance of- 

 fered to motion by a rough surface. 

 In mechanics, it is divided into sli- 

 ding, Tolling, and pirot friction. Of 

 tliese, the amount of friction with 

 the same weight is twelve to twenty 

 times greater in sliding than rolling, 

 and is intermediate in pivot friction. 

 Friction is diminished by polish, by 

 reduction of weight, by causing the 

 surfaces to be of dissimilar substan- 

 ces, as metal or wood, and by apply- 

 ing grease in sliding and pivot fric- 

 tion, but not in rolling friction. 



Whenever it is possible to convert 

 a sliding or pivot motion into a roll- 

 ing one, much is gained ; hence the 

 arrangement of friction wheels, which 

 consist of two or more wheels, sup- 

 porting a pivot or axle, and which, by 

 turning, produce a rolling instead of 

 a pivot friction. Friction wheels have 

 been recently introduced iiilo carriage 

 axles ; the axis, instead of playing in 

 the box, is surrounded with small, 

 loose rollers, v»-hich revolve with it, 

 reducing the friction to a remarkable 

 extent. 



Rigidity of cordage is also produc- 

 tive, like all kinds of friction, of loss 

 of power, and is to be guarded against 

 by using flexible ropes. The amount 

 of friction depends, in a great meas- 

 ure, on the diameter of the wheel 

 over which it passes, the extent of 

 rope in contact, and its tension. When 

 there is more than one fold, it be- 

 comes enormous, so as to be used 

 as a mechanical means for stopping 

 boats, &c. 



FRIEZE. In architecture, the cen- 

 tral portion of the entablature be- 

 tween the architrave and cornice. 

 It is plain in the Tuscan, but adorn- 

 ed in other styles. 



FRIGID ZONE. The space above 

 76i- degrees of north or south lati- 

 tude. It is scarcely occupied by any 

 plants but a few lichens. 



FRINGE-TREE. Chionantlms Vir- 

 ginica. A beautiful ornamental tree, 

 growing wild as far north as Delaware, 

 and bearing white flowers in May. 



FRINGILLID.E (from /nrt^(7/a, a 

 chaffinch.) A tribe of birds, of the 

 Passerine family, with stout, conical 



bills (conirostrcs), including the lin- 

 nets, canaries, finches, all of which 

 are devourers of grain. 



FROGS. Amphibious animals, of 

 the genus Rana. The common spe- 

 cies {li. tcmporaria and escu/eitia) are, 

 for the mciBt part, insectivorous. They 

 should not be destroyed by the gar- 

 dener, except when in great numbers, 

 as they clear his garden of slugs, 

 snails, and other small pests. The 

 green frog {csculcnta) is a great del- 

 icacy with some : the flesh of the hind 

 leg is the part eaten. 



FROG OF THE HORSE. A tri- 

 angular portion of horn projecting 

 from the sole almost on a level with 

 tiie crust, and defending a soft and 

 elastic substance called the sensible 

 frog. The sensible frog occupies the 

 whole of the back part of the foot, 

 above the horny frog and between 

 the cartilages. 



FROG HOPPERS. FROG SPIT- 

 TLE. CUCKOO SPITTLE. Small 

 insects (.Ccrcopidida) which inhabit 

 the twigs and branches of plants, 

 from which they extract so much 

 juice that the place on which they 

 are collected appears covered with 

 spittle. 



FROND. The leaves of ferns are 

 so called. 



FRONTAL. In anatomy, apper 

 taining to the forehead. 



FRONTLET. In ornithology, the 

 part of the head next the bill, usually 

 covered with bristles. 



FROST. Correctly speaking, frost 

 is ice produced by radiation. Under 

 the article Dew it has been stated 

 that in bright, clear, calm nights, all 

 objects exposed to the air become 

 cooled by radiation into the sky ; if 

 the temperature falls to 32", then 

 frost, or pellicles of ice, are produced 

 instead of dew. The word is also 

 used in common to express freezing ; 

 but, in the latter case, the air is 

 usually at or below 32° Fahrenheit, 

 whereas night frosts often occur in 

 the spring and fall when the air is 

 40° to 50 - Fahrenheit. Frosts fall ear- 

 lier on dark, rough lands tiian those 

 of a light colour and indurated con- 

 dition. 



309 



