504 L. A. Bauer — Wilde's Explication of the Secular 



the secular variation, they cannot be entered upon here. With 

 the apparatus in this condition he gets the secular variation at 

 Cape of Good Hope and St. Helena. It took 102° of differ- 

 ential motion to produce the maximum west at Good Hope. 

 As it had previously been found from the London results that 

 the annual differential motion was 22 - 5 minutes of arc, 102° 



would be equivalent to — ^— K — or 272 years. From the ob- 



1 22-5 J 



servations we get about the same interval. Hence, here we 

 have apparently a close correspondence between theory and 

 fact, albeit that there is an average difference of about 3° 

 between the two curves all along and but one point of corre- 

 spondence actually obtained. It will also be seen from the 

 diagram (A) that by starting about 50 years later, or in other 

 words, if the zero at Good Hope had occurred at the same 

 time as at London, the same maximum westerly elongation of 

 about 25° would have been obtained as at London. It looks 

 very much as though Wilde happened to strike the maximum 

 of 30° instead of 25° because about 50 years more of secular 

 variation had been ground out. But we will let Wilde have 

 this point and proceed to the next station which furnishes the 

 most satisfactory test. 



Here he had no means of knowing when the maximum west 

 would set in and what its amount would be and hence would 

 be unbiased. The last recorded observation was in 1846, 

 (23° 11' W.) indicating that westerly declination was still 

 increasing. In his table of observations, Wilde gives 26° 00' 

 W. for 1880, which, I presume, was taken from the British 

 Admiralty Chart. This is the last that he knew anything 

 about. In 1890, Mr. E. D. Preston, while on the Solar Eclipse 

 Expedition to St. Paul de Loanda, touched at Helena and 

 made magnetic observations. He got an average value of 24 , 3° 

 for the magnetic declination. Mr. Preston's observations have 

 been published in Bulletin No. 23 of the Coast and Geo- 

 detic Survey. Even without the supposititious value of 1880 

 Prof. Schott, by expressing the magnetic declination in the 

 form of a sine function, has deduced that the maximum 

 occurred about 1890. As a further check, I have examined 

 the excellent magnetic charts for 1885 by Neumayer in 

 Berghaus's Physikalischer Atlas and find for 1885, 25°, showing 

 very clearly that the maximum occurred somewhere between 

 1880 and 1890. Wilde, as will be seen from the diagram 

 (A) and his table given below,, gets the maximum west in 

 1939, or fifty years later. Hence, here he has failed, but it 

 will be noticed that in this case there is the closest correspond- 

 ence between the two curves. Let us assume now that the 

 amount of annual retardation and period of a differential revolu- 



