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The Museum Gazette 



the infection has been aerial, then prompt removal of the foliage may 

 save the tubers. There is no doubt that if the disease has shown 

 itself early in the season, the leafage being still in full vigour, the 

 removal of the latter will be very prejudicial to the crop. 



Marshall Ward speaks of the possibility of the disease being intro- 

 duced by " a stray tuber," and then spreading, of course by aerial 

 infection, over the whole field. Mr. Ward distinctly asserts the 

 possibility of the fungus wintering in the tuber. 



It is not very difficult to form an opinion as to whether any given 

 plant has been infected from below or from above, that is, from its set 

 or by its leaves. If the fungus comes from the set the whole plant 

 will shrivel and wither, the topmost leaves dying last. Under such 

 circumstances there will be discoloured spots on the leaves, but these 

 do not involve the whole leaf, and the remainder is yellow. If aerial 

 infection has occurred, the top leaves will be the first to show spots 

 and the patches will occur on leaves otherwise healthy. 



MRS. SOMERVILLE ON THUNDERSTORMS. 



We extract the following from Mrs. Somerville's well- 

 known work on the Physical Sciences. It gives an excellent 

 summary of the principal facts and may be studied with 

 interest and advantage as introductory to our own statements. 



" The contact of earth with salt and fresh water generates positive 

 electricity, and the contact of fresh and salt currents of water negative, 

 so that the ocean must afford a great supply to the atmosphere ; hence 

 thunderstorms are most frequent near the coasts. When the invisible 

 vapour rises charged with electricity into the cold regions of the 

 atmosphere, it is condensed into cloud, in which the tension is in- 

 creased because the electricity is confined to a smaller space ; and if 

 the condensation be sufficient to produce drops of rain, they carry 

 the electricity to the ground, so that in general a shower is a con- 

 ductor between the clouds and the earth. When two clouds charged 

 with opposite kinds, but of equal tension, approach within a certain 

 distance, the intensity increases on the sides of the clouds that are 

 nearest to one another ; and when the tension is great enough to 

 overcome the coercive pressure of the atmosphere, a discharge takes 

 place which causes a flash of lightning. The actual quantity of 

 electricity in any part of a cloud is extremely small. The intensity 

 of the flash arises from the great extent of surface over which it is 

 spread, so that clouds may be compared to enormous Leyden jars 

 thinly coated with electricity, which only acquires its intensity by its 

 instantaneous condensation. The rapid and irregular motions of 

 thunder clouds are probably more owing to strong electrical attrac- 



