ON COMPARINa AND EEDDCINa MAGNETIC OBSERVATIONS. 323 



by Messrs. Stewart and Carpenter of theii' last report, in which Kew De- 

 clination Disturbances are classified according to the age of the moon, 

 and in which likewise a comparison is made between declination dis- 

 turbances and wind values with the object of finding whether there is any 

 relation between these two phenomena. The fifth consists of some 

 remarks by Sir J. Henry Lefroy on disturbances near the uOrth ma"-netic 

 pole ; and the sixth, of some remarks by Mr. C. Chambers on the luni- 

 solar variation of the vertical magnetic force at Bombay. 



The Committee have drawn 2GZ. 2s., and returned to the Association 

 a balance of Vol. 18*-. They would desire their re-appointment, and 

 would request that the sum of 15?. should be placed at their disposal, to 

 be spent as they think best on the subjects mentioned in this report. 



Appendix I. Letter from Dr. Buys-Ballot to the Secretary. 



In the Second Report of the Committee on Comparing and Reducing 

 Magnetic Observations it has been said, p. 51, ' Sabine's method has done 

 good work in the past undoubtedly, but now the question has arisen, 

 Has a better been proposed ? ' 



You give some other propositions but do not mention mine, which 

 I gave in 1862 : ' Versl. der sectievergaderingen van het Prov. Utr. 

 Genootschap.' 



To this inquiry I was compelled by the very words of General Sabine, 

 preface to the ' St. Helena Observations,' page siv. : ' Until sufficient 

 •data should be obtained for the establishment of general laws regulating 

 the times of occurrence and approximate magnitude of the disturbances 

 in different parts of the globe the elimination of their influence by a pro- 

 cess similar to that adopted at the colonial observatories, or by some 

 process which should more effectually answer the purpose, must be a, 

 necessary preliminary to all precise investigations on other points.' 



Now I ventured to imagine that the first question is : How to find the 

 normal values of the declination and other elements, in order to know 

 positively what are to be considered as disturbances ? Acknowledging that 

 a distinguished philosopher such as General Sabine had great experience 

 and tact to distinguish the perturbed from the normal observations, it 

 seemed to me that when the observations of several places are brought 

 into comparison with one another, it were better to give a rule for sepa- 

 rating the disturbances. 



I proposed to take the general means of all observations without ex- 

 ception, and then to take the deviations, and to consider them all as dis- 

 turbances. Further, 1 investigate in what manner the disturbances o£ 

 different size, 0-1, 1-2, &c., minutes, occur at the various hours of the 

 day, and fix the limit of ordinary and larger disturbances for each 

 place, at that size, where the disturbances began to be distributed in 

 another manner. This method I showed to be effectual in the above- 

 named paper of 1862 in comparing the simultaneous disturbances at 

 Toronto, the Cape, St. Helena, and Hobarton. 



As now the international polar exf edition took place, and it appeared 

 necessary to calculate all these observations after the same method, 1 had 

 care to reprint this paper in the ' Archives Neerlandaises,' 1884, and to 

 submit it to the conference of the polar committee in Vienna, omitting 

 only the discussion of the observations at Toronto, St. Helena, the Cape, 

 and Hobarton, since it was only intended to show how to apply the 



Y 2 



