Dr. Mac CuLtocH on the Geology ofGleti Tilt, 329 



paper to which I have already alluded, and shall not therefore re- 

 peat it here. On the extent of their influences I have also spoken- 

 In the same place, where I have shown that the needle was sen- 

 sibly disturbed even at the distance of four feet from the ground. 

 There can be no doubt that influences of this power and extent 

 will produce local variations which if not absolutely permanent, 

 will, by affecting the other better known variations of the needle, 

 introduce errors into those observations which assume for granted 

 a certain local and temporary variation on the evidence of obser- 

 vations made at a distanc'^ from the precise point where these addi- 

 tional disturbing forces are in action. 



It is already well known to philosophers that the ordinary varia- 

 tions of the needle are not steady, either through small spaces of time, 

 or over a considerable extent of surface, and it is well known that 

 in different parts of Scotland the quantities of the variation are sub- 

 ject at any given time to considerable differences. To this cause 

 are doubtless to be attributed errors in geodesic operations which 

 have occurred even to experienced observers, where these differ- 

 ences were either neglected or perhaps not suspected. The errors 

 in General Roy's Survey of Scotland, appear to have been derived 

 at least in part from this cause. It would be interesting to know 

 how far these local differences depend on the magnetic influences of 

 masses of rock, or merely on the same mysterious cause which 

 produces the ordinary variation, acting in an unequal manner. 

 We must look to future investigations for the solution of this 

 question. In the mean time, while we are sure that such 

 irregularities actually exist, it is plain that much caution is 

 required in the use of surveying instruments, where magnets 

 are concerned either in the observations, or in the adjustment. 



Vol. III. 2 t 



