164 



SCIENCE 



[N. S. Vol. XLII. No. 1074 



effect being looked for, another change, pro- 

 portional to the square of the angular velocity, 

 was found and traced to the radial expansion 

 of the rod produced by rotation. 



The intensity of magnetization per unit 

 speed produced at the center of the iron rod 

 was about 1.5 X 10"^ c.g.s. unit per r.p.s. If 

 the rod had been rotated at the speed of the 

 earth, viz., 1/86400 r.p.s., its intensity of mag- 

 netization would have been about 2 X 10"^° 

 that of the earth, and still less if the shape had 

 been spherical. This, however, does not prove 

 that even a very large part of the earth's mag- 

 netization may not be due to the effect in 

 question, as we are entirely ignorant of the 

 magnetic properties of all substances under 

 the conditions prevailing within almost the 

 whole of the earth. Schuster has pointed out 

 that an effect of this kind may explain both 

 the mean magnetization of the earth and the 

 secular variation as well. It seems more 

 likely, however, that a large part of terrestrial 

 and solar magnetization is due to other causes, 

 such as the outward radial displacement of 

 electrons by centrifugal and thermionic action. 



A full account of the work summarized here, 

 and presented to the American Physical So- 

 ciety at its meetings of last December and 

 April, will be published in the Physical Revieiv. 

 S. J. Baenett 



The Ohio State University 



the pond-lily aphid as a plum pest^ 



One. of our best-known aphids common 

 upon various water plants is Rhopalosiphum 

 nymphwae (Linn.). This has received con- 

 siderable attention as a " semi-aquatic " species 

 which on account of the waxgland areas of its 

 body appears to be particularly adapted to a 

 life in moist localities and to suffer no incon- 

 venience from contact with water while feed- 

 ing on aquatic plants. 



An account of this species under the title 

 " A Lacustrine Aphid," by Professor T. D. A. 

 Cockerell, appeared in Science (Vol. 22, 

 p. 764) in 1905, and it is not necessary here to 



1 Papers from the Maine Agricultural Experi- 

 ment Station: Entomology No. 75. 



discuss the aquatic phases of its career, but 

 merely to call attention to another chapter in 

 the life cycle of BhopalosipJium nymphmce — 

 which proved a surprise to the writer and has 

 evidently remained unsuspected by other ob- 

 servers of this species from Linne down to the 

 present time. 



One of the most troublesome of our plum 

 aphids in Maine is a species inhabiting the 

 shoots and the ventral surface of the leaves, 

 ordinarily without causing curl or similar 

 deformation of the leaf, but exhibiting a dan- 

 gerous tendency to feed also upon the young 

 fruit itself as well as tapping the fruit stems. 



After watching this plum aphid several 

 years, and wondering where its summer home 

 might be (for it is a migratory species, leaving 

 the plum in June), it finally dawned upon the 

 writer that there were apparently no structural 

 characters to separate this from the common 

 pond lily aphid, B. nymphmw. Peculiar wax- 

 gland areas and all, the plum pest appeared to 

 be identical with the species long known to 

 science upon various water plants. 



This spring the writer made the " migration 

 test " by placing the spring migrants (alate 

 viviparous forms) from plum upon water 

 plantain, Alisma Plantago-aquatica; arrow- 

 head, Bagittaria latifolia; and cat-tail flag, 

 Typlia latifolia; which had been potted and 

 kept under laboratory control. These three 

 plants are on the approved dietary of B. 

 nympTiwcB and the plum migrants accepted 

 them all readily, and the progeny of the plum 

 migrants are perfectly content with the habitat 

 given them. 



Thus the life cycle of the ancient aphid is 

 found to include a residence upon the plum, 

 migrating thence to water plants for the sum- 

 mer and returning to the plum in the fall for 

 the deposition of the over-wintering egg which 

 provides for its spring generations upon that 

 tree. 



It is not the purpose of this note to discuss 

 the synonymy of the aphid here considered, 

 but it might be stated that it apparently exists 

 under a name more recent than nymphosw in 

 its role as a European plum pest. 



Edith M. Patch 



