Terrestrial Magnetic Intensity. 371 



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 all probability that the variation in the 

 latter should be wholly due to the indirect influence of the 

 former. We admit that the change may be due to both 

 causes conjointly; but further, if we adopt Humboldt's esti- 

 mate (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 believe 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 interval. 

 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 observations 

 cannot for a moment be sustained. 



47. Observations of dip I have not, however, neglected. 

 My instrument was a very small one (three inches diameter), 

 constructed by Mr. Robinson for the late Captain Kater, and 

 incapable of indicating such small variations as are required 

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

 hope that the small number of observations which I have ac- 

 cumulated can throw any light upon the influence of height 

 on the dip. Still these observations may fix the dip at several 

 stations with considerable accuracy, and the collation of them 

 shows 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 the isoclinal 

 lines would, even by this instrument, have been determined 

 with very considerable exactness. The following table con- 

 tains observations made with only one needle (marked No. II.), 

 the other (from having too thick an axis) having been found 

 to give much more anomalous results. 



3 B2 



