397 



was to be directed, the Report of the Committee of Physics, ap- 

 proved 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 may be convenient to reproduce these, when desiring to 

 show the degree in which the Observatories have fulfilled their contem- 

 plated 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 tem- 

 porary fluctuations are either neglected, or eliminated by extending 

 the observations over a sufficient time to neutralise their effects. 

 The other comprises the history of all that is not permanent in the 

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

 monthly, or annual change and restoration ; or in progressive changes 

 not compensated by counter-changes, but going on continually accu- 

 mulating in one direction, so as in the course of many years to alter 

 the mean amount of the quantities observed." Report, pp. 1, 2. 



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 intensity of the magnetic force, 

 require to be precisely ascertained, before the magnetic 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 constant state of fluctuation, and affected by transient and irregular 

 changes, the investigation of the laws, extent, and mutual relations 

 of these changes is now become essential to the successful prosecu- 

 tion 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 sepa- 

 rate 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 is re- 

 marked "These cannot be concluded from comparatively short 

 series of observations without giving to those observations extreme 



2 H 2 



