July 1, 1891.] 



KNOWLEDGE 



135 



be double that of tbe lower part B D C. Now, let A 

 gradually approach the shell, then the lower part B D C 

 gets larger, but the attraction of the shell is still double of 

 it ; and ultimately, when A coincides with E, the attrac- 

 tion of the shell is double of itself. E. CH.uiTKEs. 



[This is perhaps too difficult for some of the readers of 

 Knowledge, who may welcome a word of explanation. 



The attraction of the lower part of the spherical shell 

 B D C on a particle at A may be shown to be equal to the 

 attraction of the upper part B E C, which lies within the 

 enveloping cone BAG, by considering the attractions of 

 the small portions of the thin shell cut out by a cone of very 

 small angle with its vertex at A. We have M N is to m n 

 as A N is to A n, and the portion of the thin spherical shell 

 intercepted at M N is to the portion intercepted at m n as 

 A N- is to A «-. But the attractions on the particle at A 

 are inversely as the square of the distance from A. There- 

 fore, the portions intercepted on the nearer and further side 

 of the shell will attract the particle at A equally. Hence, 



the whole exterior portion B D C attracts a particle at A 

 equally with the mterior portion BEG. But this is only 

 true for a shell the thickness of which vanishes compared 

 with the distance of A from the shell. In the case of a 

 particle approaching and ultimately resting on a shell of 

 sensible thickness — we may determine the attraction by 

 considering the shell as composed of a number of very 

 thin concentric strata. In Eig. II. let S S', T T', E E' 

 represent thin concentric strata, with centre C, the particle 

 being at A. Bisect A G in D, and with centre D describe 

 the circle A S T E G. Then A S, A T, A E will be 

 tangents to the circles S S', T T' and E E', for the angles 

 A S G, A T C and AEG are all right angles, since the 

 points S T E lie on a semi-circle with diameter A G. 

 The attraction of the portion of the thin spherical 

 stratum U II' within the enveloping cone will be equal to 

 the attraction of the whole of the rest of the stratum, and 

 so for any other thin stratum T T' or S S' ; consequently, 

 for a shell of sensible thickness, the attraction of the 

 material within the small sphere A S T E C 11' T' S' will 

 be equal to the attraction of the whole of the rest of the 

 shell. 



In the case discussed by Mr. Chartres, when the 

 spherical shell is thin there will be very little attracting 

 matter within the smaller sphere described on the radius 

 of the larger sphere as diameter, but as this matter is close 

 to A the intensity of the attracting action will be con- 

 siderable, and will balance the attracting action of the rest 

 of the shell. — A. C. Eanvard.1 



THE POTATO FUNGUS. 



By J. Pentland Smith, M.A., B.Sc, &c., Lecturer on 

 Botany, Horticultural Gollege, Swanley. 



IN the " Travels and Life-History of a Fungus," 

 chronicled in the .June numljer of Knowxedge, the 

 nature of those lowly organisms termed Fungi 

 was briefly sketched. The example then selected 

 had an extremely interesting and complex life-history, 

 but the reader will probably acknowledge that the form 

 which is brought before his notice in this paper has an 

 equally romantic story to tell. Its ravages have been the 

 subject of much discussion lately in connection with the 

 distress prevalent amongst the Irish peasantry, but that 

 was not the first time that it had occupied the attention of 

 the public and Parliament in this country. In fact, to 

 quote the words of Dr. Plowright, in a lecture delivered 

 lately at the Eoyal College of Surgeons, and reported in the 

 (Jardenerx' Chronicle, " of all the plant diseases of fungoid 

 origin, there is none which has aroused more general 

 interest than that caused by the parasitic Fungus, known 

 to scientists as Plii/tniihtJwm iufofttois. This is one of the 

 very few Fungi which can claim to having eliected a change 

 in the laws of our laud. Politicians and reformers take 

 credit to themselves for having, by dint of much writing 

 and more talking, brought about the repeal of the corn 

 laws, but there are those who still say, if it had not been 

 for the Potato disease and the famine it caused in Ireland, 

 this political change would not have taken place until long 

 after it really did." 



It was in 181.3 that the first attack general over Great 

 Britain and Ireland was noticed, but more localised attacks 

 in this and neighbouring lands had been chronicled in pre- 

 vious years ; they, however, were so sUght as not to attract 

 much attention. " So momentous a calamity as befell 

 Great Britain and Ireland," says Dr. Plowright, '• by the 

 failure of the potato crop has seldom been equalled, and is 

 quite without parallel in the records of food-supply in our 

 time." 



The Potato {Solanum tuberosiun) belongs to the Natural 

 Order, or group of plants, Solmuicece. Its native place is 

 the Cordilleras of Chih, and perhaps of Peru. Its parasite 

 in this country doubtless attacked it in its native wilds. It 

 is now upwards of 300 years since it was introduced into 

 Europe, first by the Spaniards and afterwards by Ealeigh, 

 but until half a century ago no appearance of the Plnjtoph- 

 thora infcutana was noticed. We reserve a probable expla- 

 nation of this curious circumstance until the description of 

 the fungus itself. 



During July and August, especially in sultry, damp 

 weather, a pestiferous, sickening odour may be observed 

 to arise from the potato-fields. An examination of the 

 plants shows that they are evidently in a diseased state. 

 The leaves are curled up, and they and the stems have a 

 blackened appearance. This condition is the result of a 

 visit of I'lii/tojihtliura i)i/i-st(iii!<. The first indication of the 

 presence of the fungus is the appearance of dark patches 

 on the leaves, then circular or oval patches of a whitish 

 bloom arise as if by magic on the under surface of the 

 leaves. Microscopic investigation reveals the cause to be 

 of a fungoid nature. If the meteoric conditions are favour- 

 able for the further growth of the fungus the field of 

 potatoes may be a blackened, putrescent mass in a few 

 days. 



Figure I. is a semi-diagrammatic transverse section of a 

 Tomato-leaf. The Tomato (Li/vopfisinon esciili'tttiiin) belongs 

 to the same natural order as the potato, and like it. ospccialiy 

 when grown out of doors, is extremely hable to the attacks of 



