MAGNETIC DISTURBANCES AT KEW. 303 



obtained instrumentally, and there are some, I believe, where smoothing of any kind 

 is exceptional or non-existent. The diurnal inequality data being published at present 

 are thus not comparable, while it is desirable that really parallel data should be 

 available for the use of those who wish to study the general laws or to find a physical 

 explanation of the phenomena. 



The best means of securing more homogeneity may depend on the true nature of 

 the difference between days of different types. Limiting our view for a moment to a 

 particular season of a particular year, our researches show that there is no such thing 

 as a single unique type of diurnal inequality. The simplest case now conceivable is 

 that there are two extreme types, the one asymptotic for quiet days, the other for 

 highly disturbed days. As regards the second hypothetical type, there are several 

 conceivable possibilities. The inequality seen on quiet days may exist equally on 

 disturbed days, being simply supplemented by an inequality not present on ideally 

 quiet days. On the other hand, the inequality seen on quiet days may be replaced 

 to a greater or less extent by a different type of inequality on disturbed days, being 

 wholly unrepresented on the ideal infinitely disturbed day. In either case there is 

 the possibility that the difference in the phenomena on quiet and disturbed days is 

 due not to any real difference in the forces to which the diurnal inequality is ultimately 

 due, but to a difference of condition in the medium where the forces act. 



If the forces which produce the diurnal inequality on quiet days act and produce 

 the same effect on disturbed days, then the type to which highly disturbed days 

 show an asymptotic approach might be regarded as resulting from the direct super- 

 position of the ideally quiet day inequality and of an inequality which represents 

 disturbance pure and simple. The ideally quiet day inequality would then enter into 

 the inequality derived from any group of disturbed days, and by subtracting the 

 ideal quiet day inequality we should obtain an inequality representing disturbance 

 alone. It would not be really necessary to know the ideal quiet day inequality, as it 

 would be eliminated by subtracting from one another the inequalities derived from 

 any two groups of days which represent different intensities of disturbance. If we 

 then had three groups of days A, B, and C, of which A included the most quiet and 

 C the most disturbed days, the inequalities B A, C B, and C A would be of the 

 same type, but would differ in range. The range in such a difference inequality 

 would, in short, be a measure of the difference in the scale of disturbance. 



30. A serious complication in the way of direct appeal to observation arises from 

 the sunspot influence. As was shown fully in (A), there is a difference between the 

 diurnal inequalities derived from quiet days in years of sunspot maximum and 

 minimum, which is not represented by mere change of amplitude. This may mean 

 that so-called quiet days are not ideally quiet, and that in years of many and in years 

 of few sunspots they are unequally affected by the residual disturbance attaching to 

 them. This, however, is pure conjecture. The disturbed days dealt with in (C) and 

 the present paper are taken from years of very varying sunspot frequency. 



