456 MATHEMATICS AND NATURAL PHILOSOPHY. 



scope than had been contemplated by any previous institutions, ov than had been 

 provided for by the arrangements or instrumental means of any then existing es- 

 tablishment, whether national or private. Not, as previously, limited to observa- 

 tions of a single element (the declination) — or combining at the most one only of 

 the components of the magnetic force, — the instructions of the Royal Society, 

 and the instrumental means prepared under its direction, provided for the exa- 

 mination, in every branch of detail, of each of the three elements which, taken in 

 combination, represent, not partially, but completely, the whole of the magnetic 

 affections experienced at the surface of the globe, classed under the several heads 

 of absolute values, secular changes, and variations either periodical or occasional, 

 — and proceeding from causes either internal or external. To meet the require- 

 ments of inductive reasoning, it was needful the results to be obtained should 

 comprehend all particulars under these several heads, attainable by an experi- 

 mental inquiry of limited duration. That no uncertainty might exist as to the 

 objects to which, in so novel an undertaking, attention was to be directed, the Re- 

 povt of the Committee of Physics, approved and adopted by the President and 

 Council of the Royal Society, stated in a very few sentences, remarkable alike for 

 their comprehensiveness and conciseness, the desiderata of magnetical science. It 

 rnav be convenient to reproduce these, when desiring to showthe degree in which 

 the Observatories have fulfilled their contemplated purposes . — " The observations 

 will naturally refer themselves to two chief branches, into which the science of 

 terrestrial magnetism in its present state may be divided. The first comprehends 

 the actual distribution of the magnetic influence over the globe, at the present 

 epoch, in its mean or average state, when the effects of temporary fluctuations are 

 either neglected or eliminated by extending the observations over sufficient time 

 to neutralise their effects. The other comprises the history of all that is not per- 

 manent in the phenomena, whether it appear in the form of momentary, daily, 

 monthly, or annual change and restoration ; or in progressive changes not com- 

 pensated by counter-changes, but going on continually accumulating in one direc- 

 tion, so as in the course of many years to alter the mean amount of the quantities 

 observed." 



With reference to the first of these two branches, viz., the actual distribution 

 of the magnetic influence over the globe at the present epoch, the Report goes on 

 to state : — " The three elements, viz.. the horizontal direction, the dip, and the in- 

 tensity of the magnetic force, require to be pre2isely ascertained, before the mag- 

 netic state of any given station on the globe can be said to be fully determined 

 and as all these elements are at each point now ascertained to be in a con- 

 stant state of fluctuation, and affected by transient and irregular changes, the in- 

 vestigation of the laws, extent, and mutual relations of these changes is now be- 

 come essential to the successful prosecution of magnetic discovery." 



With reference to the second branch, viz., the secular and periodical variations, 

 it is observed that — " The progressive and periodical being mixed up with the 

 transitory changes, it is impossible to separate them so as to obtain a correct 

 knowledge and analysis of the former, without taking express account of and 

 eliminating the latter ;" and with reference to the secular changes in particular, it 

 narked — " These cannot be concluded from comparatively short series of ob- 

 servations without giving to those observations extreme nicety, so as to determine 

 with perfect precision the mean state of the elements at the two extremes of the 



