RECENT VALUES OF THE MAGNETIC ELEMENTS. 431 



Thus when, as usually happens, observations of an element 

 are taken at times other than those at which it just attains 

 its mean daily value, allowance should be made for the 

 diurnal inequality. Finally, irregular fluctuations in the 

 magnetic elements are not infrequent. Occasionally these 

 are very large ; for instance, Professor Adams in the Philo- 

 sophical Transactions for 1892, p. 136, refers to a rapid 

 change of over fifty-three minutes of arc as occurring on 

 one occasion in the declination at Toronto. In such an ex- 

 ceptional case, of course, a careful observer would almost 

 certainly detect the existence of a severe magnetic storm. 

 Usuallv, however, irregular fluctuations are neither so sud- 

 den nor so large as to betray their existence to an observer 

 on a survey, as he has not the proper instruments for deal- 

 ing with them. The ideal arrangement, no doubt, would 

 be to have instruments giving a continuous magnetic record 

 at all the places where absolute observations are taken, 

 but the expense is prohibitive, and fortunately the case is 

 fairly met by a comparatively small number of fixed observa- 

 tories. The secular variation, the diurnal inequality, and, 

 as a rule, even the irregular fluctuations, present very similar 

 features over a large area. 1 The curves showing the fluc- 

 tuations recorded by instruments of the same pattern at two 

 observatories within a few hundred miles of one another are 

 closely similar. If the disturbances are large and sudden it 

 would perhaps be hardly prudent to rely on the records of a 

 distant observatory for particulars. But even then these 

 records might prove of no inconsiderable use indirectly in 

 checking the fancy of an observer when working up his 

 results. 



To enter on details with respect to the magnetic records 

 for one year of even a single observatory would occupy too 

 much space. It is possible, however, to supply in brief 

 compass a table of the mean annual values of the elements 

 at a number of observatories, and this is done here with the 

 hope that it will prove both useful and interesting. In one 



1 Cf. Professor W. G. Adams, B. A. Reports, 1880, p. 201, and 1881, p. 

 463, also Mr. W. Ellis, Proceedings of Royal Society, vol. lii., p. 191, 1892. 



