124 JOHN S. BARLOW 



with geographic latitude. Since, according to these authors, the isolines for 

 winter solar radiation follow the lines of geographic latitude fairly closely, it 

 is not clear that the relationship observed between December solar radiation 

 and distribution of multiple sclerosis by birthplace would prevail in a similar 

 manner for other areas of the world, for geographic latitude itself does not 

 appear to be a good correlate when data for the disease from different areas 

 of the world are examined (Barlow, 1960). It may well be, however, that 

 the geographic distribution of the disease is deteiTnined by several factors, 

 and the importance of each of these may vary in different areas. 



The possibility that geologic factors, perhaps in trace elements, may have 

 a role in the distribution of multiple sclerosis recently has been reiterated by 

 Warren (1959). 



Latitude Distribution of Multiple Sclerosis 



In the United States in recent years. Dr. Leonard T. Kurland of the Epi- 

 demiology Branch of the National Institute of Neurological Diseases and 

 Blindness has been particularly concerned with surveys on multiple sclerosis, 

 and it was as a result of his epidemiologic summary at the First International 

 Congress of Neurological Sciences in Brussels in 1957 that the present ap- 

 proach had its origin. At that congress, Kurland presented data for North 

 America which indicated that the frequency of the disease is strongly 

 dependent on latitude, and he suggested that any satisfactory explanation of 

 the etiology must take the geographic factor into account (Kurland et al., 

 1957), a view also held by other observers (McAlpine et al., 1955). 



The variation of the disease with latitude particularly interested me, and I 

 undertook to determine if there were any similarity between this latitude 

 effect and the intensity of cosmic rays, whose distribution is well known to 

 be dependent in part on latitude (Barlow, 1959). 



The variation of cosmic ray flux is determined by the earth's magnetic field 

 and therefore is related to geomagnetic latitude rather than to geographic 

 latitude. A map indicating geomagnetic latitude in relation to geographic 

 latitude is reproduced in Fig. 1. The lines of constant geomagnetic latitude 

 are skewed with respect to those of constant geographic latitude, derived 

 from the fact that the earth's magnetic axis is inclined at an angle of about 

 10° with respect to its axis of rotation. For the eastern United States, the 

 geomagnetic latitude for a given location is about 10° greater than its geo- 

 graphic latitude; for western Europe, the two are approximately the same, 

 and for eastern Asia, the geomagnetic latitude for a particular location is 

 about 10° less than its geographic latitude. It is apparent then that the two 

 latitudes may differ from one another by an amount up to plus or minus 10°, 

 a total rans;e of about 20°. 



