of Terrestrial Magnetism, 389 



sphere. That such a series of observations might be carried on 

 with due precision, a military service was organized (under the 

 general direction of Colonel Sabine), in which the observers suc- 

 ceeded each other day and night. They were taken from the 

 corps of Artillery, selecting among the most intelligent those 

 who showed most taste for the work. The staff of each observa- 

 tory consisted of at least four non-commissioned officers and two 

 soldiers, under the direction of a captain fully instructed in the 

 duties to be carried on. The series was maintained for several 

 years ; in some cases more, in others less, but nowhere for less 

 than five years. 



If the quahty of the instruments, the system of distribution of 

 the observatories, and above all the moral character of the nation, 

 constant and assiduous in carrying through whatever it under- 

 takes, be considered, it will be owned that here was ground for 

 expecting the manifestation of any discoverable laws, and that 

 such data deserve full confidence as a basis of serious discussion. 

 But undigested masses of figures are of small profit to science, 

 and remain always a useless incumbrance in libraries and archives, 

 if they are not reduced, examined, and discussed. 



This second part of the work, perhaps not less laborious than 

 the first, and requiring more ability, was also confided to the 

 care of Colonel Sabine ; and under his direction the numerous 

 calculations of the reductions have been made, and published in 

 nine large quarto volumes, at the expense of the British govern- 

 ment, and which have been liberally distributed ; the last was 

 published a few months ago, and more volumes are in course of 

 publication. 



Besides the numerical calculations. Colonel Sabine has given, 

 in the Introductions to these volumes, graphical representations 

 of the variations at each place, and has compared the results 

 from the different parts of the world, in order to deduce from 

 them the most important conclusions. The whole has been 

 executed with surprising sagacity, and at the same time with 

 the greatest reserve as to theoretic hypotheses, a thing diffi- 

 cult in such matters, but strictly Tcquisite where the funda- 

 mental data of a science have to be fixed. As this work is the 

 most extensive of those hitherto published in tabular reductions, 

 and the most important in respect of the points of the globe 

 occupied, so it will be our principal point of departure in the 

 deduction of laws*. We shall not however omit putting under 



* The work of reduction is of the highest importance, and it is necessary 

 that it should be executed by those who have made or superintended the 

 observations, to avoid the multifarious doubts which arise in its course. 

 But in such reductions it is not sufficient to give annual and monthly 

 means ; it is also necessary to give graphical constructions, which are of 



