Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/337

 paper to which I have already alluded, and shall not therefore repeat it here. On the extent of their influences I have also spoken in the same place, where I have shown that the needle was sensibly 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 observations made at a distance from the precise point where these additional disturbing forces are in action.

It is already well known to philosophers that the ordinary variations 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 subject 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 differences 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 fixture 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.