56 
MR. A. W. RUCKER AKD DR. T. E. THORPE OK A MAGNETIC 
In our opinion, it would ju'obably have been better if a larger number of observers 
bad been eunao-ed in the task, so that it could have been finished iu a shorter time. In 
fact, we originally made a proposal in this sense which was, we believe, brought before 
the Kew Committee of the Royal Society. It was considered that the objections to 
the employment of many instruments and observers were sufiicient to j^revent the 
acceptance of such a scheme. With proper organisation, we believe that the errors thus 
introduced would have been much less serious than those due to the uncertainty as to 
the true secular corrections at any particular station. As, however, it was more 
important that the survey should be made than that it should be made under the 
best possible conditions, we have in undertaking the task ourselves devoted most of 
our vacations and spare time to it, so that there should be as little delay as possible, 
and have collected all the facts which tlirow light on the value of the secular 
corrections. 
Although the re-determination of the lines of equal magnetic Declination, Force, and 
Dip has been the main object of our investigation, a number of other questions 
have naturally come under consideration. We have made some alterations in the Kew 
magnetometer which have been described to the Physical Society (‘Phil. Mag.,’ 
August, 1888, p. 122) ; the method of presenting the results of the experiments has 
been modified so as to afford a greater test of their accuracy ; the validity of the 
method of correcting for diurnal variation and disturbance, especially at places far 
distant from the base station, has been reconsidered, and finally, and perhaps chiefly, 
we have given more attention than our predecessors have done to the distribution and 
causes of “ local magnetic attraction.” In view of the difficulties caused by such 
disturbances, we have taken special pains to indicate the position of our stations as 
accurately as possible. This has been done, not merely by verbal description, but by 
taking out the latitudes and longitudes from the inch Ordnance maps or the Admiralty 
charts, with far greater accuracy than is necessary for the calculations in which these 
quantities are afteinvards employed. For similar reasons we have selected, when 
possible, public parks, open commons, or other situations which are likely to be still 
available when the survey is repeated. It must, howevei', be remarked that, at some 
places which were not included in our original programme, observations have been 
made mainly because a favourable opportunity presented itself, and that, in such cases, 
it was not always possible to exercise the same care in the selection of a site. 
Epoch. 
The epoch of the survey is January 1, 1886, to which date all the observations 
have been reduced. 
Iiistruments. 
The survey of Scotland was mainly made with a set of instruments which belong to 
Professor Rucker. They are a Kew Magnetometer by Elliott Bros., No. 60, and a 
