POL 



576 



POL 



joints and crevices of a wall with mortar 

 or cement. 



POINT NET, or LACE. Lace worked with 

 the needle, now superseded by bobbin- 

 net. See POINT. 



POI'SON. Any substance which in a 

 particular manner deranges the vital 

 functions and produces a mortal effect, if 

 not counteracted. Substances of this 

 sort are generally deleterious in certain 

 doses; for, in small doses, the most active 

 form the most valuable medicines. There 

 are, however, some which are deleterious 

 in the smallest doses, as hydrophobic virus; 

 others are not deleterious when taken 

 into the stomach, which are highly so 

 when taken into the lungs, or applied to 

 an abraded surface, as sulphuretted hy- 

 drogen. 



POI'TREL, Vr.poitrail. A graving tool. 



POLAC'CA. A peculiar three-masted vessel. 



POL'ACRE (Sp. and Fr.). A vessel with 

 three masts, each of one piece, sr that 

 they have neither tops, caps, nor cross- 

 trees to their upper yards: used in the 

 Mediterranean. 



POLAR': TT. The opposition of two 

 forces in bodies, or that quality of a body 

 by virtue of which peculiar properties 

 reside in certain points : usually, as in 

 magnetised and electrified bodies, pro- 

 j>erties of attraction and repulsion, or the 

 power of taking a particular direction 

 whea freely suspended. Thus, we speak 

 of tbi polarity of the magnetic needle. 

 S<?MAC.NET. 



POLARIZA'TUVS. The communication of 

 polarity. By the polarization of light and 

 heat is meant the change produced upon 

 their agencies by reflection or refraction 

 at certain angles, or by refraction in cer- 

 tain crystals, whereby they acquire the 

 property of exhibiting opposite effects in 

 planes at right angles to each other. 

 Thus, when a ray of light is passed 

 through a crystal of Iceland spar, it is 

 separated into two portions of equal in- 

 tensity ; but if each of these rays be fur- 

 ther acted upon by a second crystal, it is 

 found that each of them has suffered a 

 physical change, and is not affected by 

 the second crystal in all positions, as it 

 was by the first. The same applies to 

 other media besides Iceland spar, and to 

 heat as well as light. The discovery of the 

 polarization of light is due to Huygens, 

 and that of heat to Prof. Forbes, of Edin- 

 burgh. 



POLE. In physics, the extremities of the 

 axis about which a body revolves, or is 

 supposed to revolve : from xo).iu, to turn. 

 Thus, the extremities of the axis of the 

 earth are called its poles, and the points 

 opposite are the poles of the world. In a 

 sphere, the polos are 90 distant from the 

 plane of a circle passing through the cen- 

 tre perpendicular to the axis. Thus, the 



zenith and nadir are the poles of the 

 horizon. 



POLE-AXE. A description of hatchet, 

 with a handle about 15 inches ions, and a 

 point or claw bending downwards from, 

 the back to its head. It is principally 

 used in actions at sea, to cut away the 

 rigging of the enemy attempting to board. 

 Sometimes it is thrust into the side of an 

 enemy's ship in attempting to board, and 

 is therefore called the boarding -are. 



POLE'CAT. The Mustela putorivs, Lin., 

 the terror of poultry-yards and warrens. 

 It is the most sanguinary of all the weasel 

 tribe. See PUTORICS. 



POIEMO'NIUM, Greek Valerian. Agenus 

 of perennial plants. Pentandria MOHO- 

 gynia. Xame from iraXeiwj , war ; because, 

 according to Pliny, kings had contended 

 for the honour of its discovery. Jacobs- 

 Ladder (P. cceruleum) is the only British 

 species. 



POLEM'OSCOPE : rroXtfM; , war, and 

 O-ZOKIU, to view. A reflecting perspective 

 glass, invented by Hevelius, who com- 

 mended it as useful in sieges, &c., for dis- 

 covering what the enemy is doing, while 

 the spectator lies hidden behind an ob- 

 stacle. The opera-glass is the polemo- 

 scope somewhat improved. 



POLES or A MAGNET. Points in a magnet 

 where the intensity of the magnetic force 

 is at a maximum : one of these attracts 

 and the other repels the same pole of an- 

 other magnet. 



POLE'-STAR. A star of the second magni- 

 tude, the last in the tail of Ursa Minor. 

 It is only 1 41' from the true pole, con- 

 sequently never sets, and is therefore of 

 great importance with navigators in the 

 northern hemisphere. 



POL'ICT. In insurance, the instrument by 

 which a contract of indemnity is effected 

 between the insurer and the insured : the 

 writing containin? the terms or conditions 

 of a contract of insurance. Policies are said 

 to be rained when the goods insured are 

 valued at prime cost, and open when the 

 goods are not valued. There are policies 

 of marine insurance, of life insurance, &c. 



POLITICAL ARITH'METIC. The art of 

 reasoning by figures on matters relating 

 to a nation, its revenues, value of lands 

 and effects, produce of lands, manufac- 

 tures, population, &c. &c. 



POLITICAL ECOIIOMY. The administra- 

 tion of the revenues of a nation, or the 

 management and regulation of its re- 

 sources and productive property and 

 labour. Political economy comprehends 

 all the measures by which the property 

 and labour of a country are directed ir. 

 the best manner to the success of indivi- 

 dual industry and enterprise, and to tho 

 public prosperity. It is now considered 

 a sci<r.ce. 





