2(j PROFESSOR FORBES'S EXPERIMENTS ON 



cxrrves from the two partial sets of curves, we also increase the probability of ac- 

 curacy, since intensity is likely to be so much oftener observed than dip ; that the 

 lines of equal horizontal intensity wiU 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 moun- 

 tain chains may cause inflections in the general course of the curves, and local 

 attractions produce occasional anomalies, yet the general variation 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 experi- 

 ments 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 inten- 

 sity may not be due to a change in the total intensity, but only of the dip. To 

 this we would reply, that no reason can be assigned why the dip should more 

 naturally vary than the intensity ; and that it is contrary to aU probability that 

 the variation in the latter should be whoUy due to the indirect influence of the 

 former. We admit that the change may be due to both causes conjointly ; but 

 farther, if we adopt Humboldt's estimate (I quote from a reference which I have 

 been unable to verify), which assigns a diminution of 2'.5 of dip for 1000 feet of 

 extent, we should have an apparent increase of horizontal intensity, if the total 

 intensity remained constant. The second objection to which I alluded, I beheve 

 no one accustomed to treat such problems will apply to my observations after 

 due examination, namely, that though three stations be in one straight line and 

 equidistant, the elevated station being in the centre, we can draw no conclusion 

 as to the variation of the intensity by comparing the extreme observations with 

 the middle one, because the dip may have altered in the inteiwal. We may indeed 

 have, by a strange accident, a solution of continuity which might produce this 

 effect in a single instance, but its capability of affecting a whole series of obser- 

 vations cannot for a moment be sustained. 



47. Observations of dip I have not, however, neglected. My instrument was 

 a verj^ small one (three inches diameter), constructed by Mr Robinson for the 

 late Captain Kater, and incapable of indicating such small variations as are re- 

 quired to fix with great accuracy the lines of equal dip. Nor can I hope that the 

 small number of obsei-vations wliich I have accumulated can throw any light 

 upon the influence of height on the dip. Still these obseiwations may fix the 

 dip at several stations with considerable accuracy, and the collation of them shew 

 that tolerable precision may be attained even with an instrument of very small 

 dimensions. Had the observations been as much multiplied as those of intensity. 



