546 
MR. A. W. RUCKER ARD UR. T. E. THORPE OR" A MAGNETIC 
kingdom, thereby introducing small discontinuities, and that in others the equations 
contain periodic terms. It would, no doubt, have been possible to smooth out these 
irregularities, but we adhere to the rule we have throughout followed of not in any wav 
modifying the terrestrial isomagnetics, which have been adopted as most closely 
representing the general form of the district curves. 
It is difficult to devise any conclusive test to ascertain whether, when these tables 
are used, there is still in any region a residual error, which makes the stations of 
either survey peculiar by relatively great Vertical Force Disturbances. The best that 
occurs to us is to compare the Vertical Force Disturbances at the 25 repeat stations 
when referred to the mean of the two sets of terrestrial lines. 
In this case, the results are quite independent, and should be affected only by the 
error of experiment, including in that term, (l) errors due to slight changes in the 
positions at which the observations were made, and (2) errors due to any real change 
in Disturbing Force which may conceivably have occurred. 
In Table XXIII. the Vertical Disturbing Forces calculated from the two surveys 
are given in terms of O'OOOl metric or O'OOOOl C.G.S. unit, together with their 
differences, and the means taken with and without reference to sign in each 
district:— 
