1879.] JRejiort to the Committee on Solar Physics. 107 



Observatory, which one of us had received through the kindness of the 

 Kew Committee. In these the daily ranges of the magnetic declina- 

 tion are given after excluding disturbed observations by the process of 

 Sir E. Sabine. The daily ranges are given in inches, and they denote 

 the differences between the greatest and least values of each day's 

 hourly tabulations from the curve, disturbances (as already mentioned) 

 being excluded. These records extend from the beginning of 1858 to 

 the end of 1873, embracing in all 16 years' observations. 



4. Let us now proceed to describe the method which we pursued in 

 our search for the natural inequalities of magnetic declination. We 

 may illustrate this by a well-known case. Suppose that we had in our 

 possession extensive records of the temperature of the earth's atmo- 

 sphere at some one place in middle latitudes, and that, independently 

 of astronomical knowledge, we were to make use of these for the pur- 

 pose of investigating the natural inequalities of terrestrial temperature. 

 We should begin by grouping the observations according to various 

 periods taken, say, at small but definite time-intervals from each other. 

 Now, if our series of observations were sufficiently extensive, and if" 

 some one of our various groupings together of this series should cor- 

 respond to a real inequality, we should expect it to exhibit a well- 

 defined and prominent fluctuation, whose departures above and below 

 the mean should be of considerable amount. Suppose, for instance, 

 that we have 24 points in our series, and that we group a loug v 

 series of temperature observations in rows of 24 each, the time- 

 distance between two contiguous members of one row being one 

 hour. The series would thus represent the mean solar day, and we 

 should no doubt obtain from a final summation of our rows a result 

 exhibiting a prominent temperature fluctuation of a well-defined 

 character, which we might measure by simply adding together all the 

 departures of its various points from the mean, whether tliese points 

 lie above or below ; in fine, by obtaining the area of the curve which is 

 the graphical representation of the inequality above and below the 

 line of abscissas taken to represent the mean of all the points. 

 Suppose next that, still keeping to rows of 24, we should make the 

 time interval between two contiguous members of a row somewhat 

 different from one hour, whether greater or less, we should now in 

 either case obtain a result exhibiting, when measured as above, a much 

 smaller inequality than that given when the interval was exactly one 

 hour, and it is even possible that if our series of observations were 

 sufficiently extensive, we should obtain hardly any traces of an inequality 

 whatever. In fine, when each, row accurately represented a solar day 

 the result would give an inequality of large amount, but Avhen each 

 row represented a period either slightly less or greater than a day the 

 result would be an inequality of small amount. 



5. We should by this process, after bestowing enormous labour in 



