LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 33 



force possibly prevailing at the time of observation ; and, 2nd, they may not be true 

 measures of the magnetic elements corresponding to the geographical position in 

 which the observations are made, by reason of those local disturbing magnetic in- 

 fluences which are included under the name of station error. 



For the first of these sources of error a remedy is presented, whenever the obser- 

 vations can be made in connexion with those of a fixed magnetic observatory, situated 

 within such distance that the magnetic elements are subject to the same periodical 

 and irregular variations. The particular advantage possessed by the absolute deter- 

 minations of the fixed observatories, — that of being mean values of the quantities 

 sought, — may thus be indefinitely extended. 



Against the more formidable evil of station error the connexion with a fixed obser- 

 vatory affords the magnetic traveller no security ; nor can it furnish him with a cor- 

 rection, — for to error from this source the absolute determinations of fixed observa- 

 tories are themselves no less liable ; and no continuance, or frequency of repetition at 

 the spot itself, will lead to its discovery or assign its correction. The magnetic survey 

 of the British Islands, and more especially of its Scottish and Irish portions, has 

 shown that such disturbances are not confined to localities, which, like Otaheite, consist 

 chiefly of volcanic rocks, but may exist unsuspected and productive of error of 

 serious amount, wherever the igneous rocks rise through, or approach the superficial 

 soil. It is this source of error which presents a practical difficulty to the determina- 

 tion of the elements of the theory of terrestrial magnetism from exact observations 

 at a few chosen positions on the globe. The remedy is to be found in the combina- 

 tion of fixed magnetic observatories and magnetic surveys ; the observations of the 

 survey being based on and executed in concert with the regular observations of a fixed 

 observatory ; the country surveyed being also sufficiently extensive to neutralise di- 

 strict anomalies, as well as those of a more local nature. The observations of the 

 survey, corrected to mean determinations by their connexion with those of an ob- 

 servatory, and combined in the manner described by Mr. Lloyd in the third section 

 of the Survey of the British Islands, will furnish in their turn the correction for the 

 station error, if any, of the fixed observatory. 



Total Intensity. — From the value of the total intensity at Otaheite as now deter- 

 mined by Captain Belcher, we learn that the southerly inflection of the isodynamic 

 lines, in and about the meridian of the Society Islands, — which was pointed out as one 

 of the characteristic features of the general configuration of those lines in the southern 

 hemisphere*, — is even more strongly marked in the latitude of those islands than I had 

 ventured to draw it, under the circumstances of the unusual discordance in the only 

 observations which we then possessed. 



* Seventh Report of the British Association, p. 73. 

 MDCCCXLI. F 



