370 Professor Forbes's Experiments on 



the variation in the time of an oscillation would be only half 

 as much. 



4S. The smallness of the variation fully explains the diffi- 

 culty of ascertaining its existence from a very limited number 

 of observations. It is hoped that, notwithstanding the im- 

 perfection of the instruments, the extent of the induction will 

 entitle the result to some confidence. By adding together 

 the elevations of the distinct stations contained in Table VII. 

 it will be found that the aggregate of the heights to which I 

 have ascended amounts to above 160,000 feet, or more than 

 thirty vertical miles. 



% 5. On the Magnetic Dip, 



44. Although the horizontal magnetic force be only a sort 

 of mathematical abstraction, and bears no direct relation to 

 the earth's action until the effect of dip is considered, we do 

 not therefore think it improper to be made a separate subject 

 of inquiry. From the projected lines of equal horizontal in- 

 tensity and of equal dip, the lines of equal total intensity are 

 deducible. The two elements may therefore be made the sub- 

 ject of distinct inquiry; and though these elements are pro- 

 bably in a condition of continual change, yet, considering the 

 present errors of observation, any moderate lapse of time be- 

 tween the formation of these curves will not be productive of 

 serious anomalies. By deducing the total intensity curves 

 from the two partial sets of curves, we also increase the pro- 

 bability of accuracy, since intensity is likely to be so much 

 oftener observed than dip; that the lines of equal horizontal 

 intensity will be better determined than if those points alone 

 were used where the dip was also observed ; and thus the 

 whole acquires additional consistency. 



45. The general relations of dip and horizontal intensity 

 have been pointed out in the excellent charts of Hansteen. 

 Though it is very probable that mountain chains may cause 

 inflections in the general course of the curves, and local at- 

 tractions produce occasional anomalies, yet the general varia- 

 tion of one or other quantity is always graduated ; and though 

 an insulated observation may be spoiled by an abrupt change 

 in either element, the conclusion from a series of experiments 

 cannot be so affected. 



46. I state this chiefly to meet two objections to conclusions 

 from experiments of the kind I have detailed, which have at 

 different times been urged. The first is, that the influence 

 of height (for example) upon the horizontal intensity may not 

 be due to a change in the total intensity, but only of the dip. 



