POTATO 



POTENTIAL 



357 



of the fungus, which at last bursts forth in little 

 cushion-shaped tufts loaded with fructification. 

 Wet Rot differs from dry rot in the tubers 

 becoming soft and rotten instead of hard and dry, 

 and is always characterised by the presence of a 

 fungus referred by Fries to his genus Periola, but 

 which Berkeley regards as another form or stage of 

 the same fungus which causes or is inseparably con- 

 nected with dry rot. Both dry rot and wet rot have 

 often been observed along with the potato disease, 

 which, however, is always characterised by the pres- 

 ence of another peculiar fungus. Peziza postuma 

 has occasioned heavy losses, chiefly in Ireland, by 

 destroying the leaves before the crop has matured. 

 See books on potato-culture by Pink (1879), Cox 

 (1880), Fremlin (1883), and Ward (1891), and on 

 the potato-blight by Bravender ( 1880). 



Potato, SWEET. See SWEET POTATO. 

 Potato-beetle. See COLORADO BEETLE. 

 Potato-fly (Anthomyia tuberosa), a dipterous 

 insect of the same 

 genus with the 

 Radish-fly, Cab- 

 bage-tly, Turnip-fly, 

 &c. In its perfect 

 state it is very like 

 the House-fly. The 

 maggots are often 

 abundant in bad 

 potatoes in autumn, 

 and are different 

 from the maggots 

 of the House-fly, 

 being horny, spiny, 

 bristly, and tawny ; 

 the long tail ending 

 in six long bristles. 

 Potato-fly (Anthomyia, tuberota): The pupa is very 

 1, Larva, or maggot, natural ize ; like the larva. 

 2, larva magnified ; 3. Potato-fly. The p ota tO-frog 



Fly (Euteryx solani, 



Curtis) and the caterpillar of the Death's-head 

 Moth (Ac/ierontia atropos, Linn.) feed on the 

 leaves and stems of potatoes, but rarely do serious 

 damage. 



Potchefstroom, a town in the south of the 

 Transvaal, South Africa, 105 miles SW. of Pretoria. 

 Pop. 2000. 



Potemkin, GREGORY ALEXANDROVITCH, the 

 most celebrated of the Empress Catharine II. 's 

 favourites, was born near Smolensk on 16th Sep- 

 tember 1739, the descendant of a noble but im- 

 poverished Polish family. Having entered the 

 Russian army, he managed (1762) to attract the 

 notice of the czarina by his handsome face and 

 athletic figure ; he was attached to her house- 

 hold, and in 1774 was preferred as her recognised 

 favourite. From 1776, when the Emperor Joseph 

 of Austria made him a prince of the Holy 

 Roman Empire, till the year of his death he 

 was the director of the Russian policy in Europe. 

 It was at his instigation that the Khan of the 

 Crimea put himself (1783) under Russian pro- 

 tection. Four years later Catharine paid a visit 

 to his government in the south, and the ' hoax ' 

 which he then played off on his sovereign is 

 described by De Segur (Mf moires). He caused 

 an immense number of wooden painted houses to 

 be constructed, and grouped into towns and villages 

 along the route the czarina was to take, and hired 

 people to act the part of villagers, merchants, 

 tradesmen, and agriculturists, engaged in their 

 various pursuits. The czarina's vanity was hugely 

 gratified at the seeming improvements of the 

 country under her rule, and she covered Potemkin 

 with titles and honours. Almost immediately 



after this a war broke out with the Turks, and 

 Pptemkin was placed at the head of the army, 

 with SuwarofF serving under him. Otchakoff was 

 taken after a terrible siege, and Suwaroft' won the 

 great fights of Bender and Ismail of all of which 

 Potemkin reaped the credit when he entered St 

 Petersburg in triumph in 1791. That same year 

 he was seized with sudden illness whilst travelling 

 l>etween Jassy and Otchakoff, and died October 

 15, and was buried at Kherson. He was a man of 

 considerable ability in court intrigue and states- 

 manship ; his skill as a general lias been both 

 affirmed and denied. Personally he was licentious, 

 coarse in his habits, and utterly tyrannical and 

 unscrupulous ; in spite of his lavish extravagance 

 he heaped up an immense fortune. 



See Afemoirt ( Lend. 1812), and the Life in German by ' 

 his secretary Saint-Jean ( new ed. Karlsruhe, 1888 ). 



Potential, in dynamical science, is a quantity 

 of peculiar importance. Its value, as a mathe- 

 matical function in the theory of attraction, was 

 recognised by Laplace in the Mecanique Celeste. 

 The name was, however, given by George Green 

 (1793-1841) in 1828, when its broad dynamical 

 significance was for the first time explicitly 

 stated and powerfully developed. The theory of 

 the potential, in fact, is co-extensive with the 

 dynamics of what are known as Conservative 

 systems. When such a system is made to pass 

 from one configuration to another, the work 

 done against the forces of the system depends 

 only upon the initial and final configurations, 

 and in no way upon the particular series of 

 changes by which the passage is made. For 

 instance, the work done against gravity in lifting 

 a given mass to a height of 500 feet is exactly the 

 same whether the mass is lifted vertically up, by 

 a balloon, say, or more laboriously taken up thu 

 gentle slope of a hill. The earth and the mass 

 form, so far as gravitation is concerned, a con- 

 servative system. Practically, however, in drugging 

 a mass up a slope a certain amount of work, gi enter 

 or smaller according to circumstances, must be 

 done against friction, and this will depend upon the 

 character of the course taken. We know that the 

 work so done is lost and cannot be recovered in 

 dynamic form (see ENERGY). These forces are in 

 short dissipative, and so far as their action is con- 

 cerned the system is not conservative, and the 

 theory of the potential does not apply. A little 

 consideration will show that when the forces are 

 functions of distances only the system will be 

 conservative. Such forces then have a potential ; 

 and, although this does not exhaust all types of 

 force-systems which have a potential, it includes 

 all that are certainly known to occur in nature 

 around us. The force of gravitation and the force 

 between electrified or magnetised bodies evidently 

 belong to the category just descrilied. In all such 

 cases the potential at any point in the field of 

 force is a definite function of the position, a 

 mathematical expression having for any particular 

 case a definite value, such that the difference of the 

 potentials of two points measures the work done 

 in carrying unit quantity (of matter, electricity, 

 magnetism, &c. ) from the one point to the other 

 (see ELECTRICITY for some further properties of 

 the potential). If we take the two points very 

 close to eacli other, we see at once that the 

 small difference of the potentials must equal the 

 product of the average force into the corresponding 

 small distance. Thus, in the notation used in the 

 article Calculus (q.v.), we have AV = SA, where 

 V is the potential, S the force, and AS the 

 small distance. Hence S = dVjds or the force in 

 any direction is numerically equal to the rate of 

 change of the potential per unit-length in that 

 direction. When the potential is known a simple 



