460 



SCIENCE 



[N. S. Vol. XLII. No. 1083 



two principal moments of inertia of the orbital 

 system. In the ease under consideration 

 B^M/2ai. Eliminating B and M from (2) 

 we get 



r = — lisma -2— { 1 + p, - cos a I .'' 

 e \ Z a ) 



(3) 



If we divide this expression by — jx. sin a we 

 shall, as in the case of an ordinary magnetic 

 field, get the intensity sought, viz.. 



^ = 2Xl + 15eos„).3 



(4) 



The magnitudes of O experimentally attain- 

 able are so small in comparison with a> that 

 the second term is always negligible. 



If we assume that e/m has the value 

 ( — LW X 10') ordinarily accepted for the 

 negative electron in slow motion, and put 

 = 2'7rn where n is the speed of the rod in 

 revolutions per second, we get for the intensity 

 per unit speed 



Bin = - 7.1 X IO- 



CS) 



This is the maximum magnitude possible; if 

 some or all of the positive ions also have 

 orbital motion, H will be smaller in magni- 

 tude than indicated by (4), but wiU stiU be 

 proportional to €1'. The experimental value of 

 JS/n was, within the accidental error, one half 

 that given by (5). 



S. J. Baenett 

 The Ohio State University 



the transmission of potato mosaic through 

 the tuber 



Mosaic of the potato is very prevalent in 

 Bermuda on the Bliss Triumph and is the 

 cause of serious losses to the growers, as the 

 yield of affected plants is reduced from 10 to 

 75 per cent., and often a field wiU have a large 

 proportion of plants with this disease. 



An inspection made in July, 1914, of the 



2 This equation also follows immediately from 

 Maxwell's equation by putting in the conditions 

 here assumed. 



3 The first term of this equation has been given 

 previously, by Einstein and de Haas, but was ob- 

 tained incorrectly, equations for a molar magnet 

 instead of a molecular magnet being employed. 



fields on Long Island in which stock was being 

 grown for shipment to Bermuda for seed pur- 

 poses showed the almost general presence of 

 mosaic on the Bliss Triumph. The same con- 

 dition existed in many fields of Bliss Triumph 

 in Maine, where the stock for Long Island is 

 obtained. These general facts strongly indi- 

 cated that the mosaic of potatoes was trans- 

 mitted by the tubers, in the first case from 

 Maine to Long Island and in the second gen- 

 eration from Long Island to Bermuda. 



There was, however, no evidence in the lit- 

 erature on potato growing to support this 

 view. Dr. W. A. Orton, in Bulletin 64 of the 

 United States Department of Agriculture, on 

 '■ Potato Wilt Leaf Eoll and Allied Diseases," 

 writes, " it is not improbable that mosaic is 

 transmitted by the tubers " adding, however, 

 that no experiments had been undertaken that 

 had conclusively proved this. Experiments 

 were consequently conducted at the agricul- 

 tural station in Bermuda with a view to se- 

 curing definite information on this point. 



Through the courtesy of Drs. I. E. Melhus 

 and L. 0. Kunkel, of the Bureau of Plant In- 

 dustry of Washington, tubers from selected 

 hills of healthy and mosaic parents were ob- 

 tained from a field at Van Buren, Maine, that 

 was visited by the writer in September, 1914. 



The tubers obtained from Van Buren were 

 planted in Bermuda in November, 1914, in 

 duplicate rows, and the result showed in a stri- 

 king manner in January, 1915, that the mosaic 

 of potatoes is transmitted through the planting 

 of tubers from mosaic parents : 



No. of Tubei Selected Percentage of 



Plants from Mosaic Plants 



A. 200 Healthy parents nil 



(4 or 5 doubtful) 



B. 200 Stock on the gQ 



market 



C. 200 Mosaic parents 100 



The yield from the plants affected with 

 mosaic was less than haK that of the healthy 

 stock. 



Mosaic of the potato is undoubtedly one of 

 the serious potato problems that have escaped 

 the notice of the practical farmer and that 

 have until recently received little attention 

 from scientific workers. To growers of the 



