28 KENNETH HARTLEY 
small number of gravity measurements would give the anomaly curve with 
a fair degree of accuracy but the same number of ordinates to the gradient 
curve would leave it still extremely uncertain and the results consequently 
very difficult to interpret and the interpretation of very doubtful validity. 
While the new instrument is still in the experimental stage and there will 
undoubtedly be improvements in details of construction, it is not probable 
that it can ever be used effectively by an unskilled operator. The method of 
operation is extremely simple but it seems to be necessary for the operator 
to have had some kind of experience with precision measurement in order to 
guard against deceptive appearances and be sure that the instrument is 
really in equilibrium when the reading is made. 
172 
