The solar eclipse of August 30, 1905, and magnetic phenomena

I have read with interest Mr. Charles Nordmann's contribution 1 to the study of the effects produced on the magnetic declination by the total solar eclipse of August 30, 1905. Whilst appreciating, as it deserves, Mr. Nordmann's ingenuity, I must confess that the facts which he advances, p. 23, seem to me to be inconclusive, and to bear most probably a different interpretation from that which he supposes. Before explaining my reasons for this view, I should like to make some preliminary remarks. Mr. Nordmann's curves, p. 22, have no claim to acceptance except as the embodiment of the figures quoted on p. 20. The unit of force employed is said to be only 0.14 γ (γ = 1 × 10 ‐5 C. G. S.). Of the five stations dealt with, Stonyhurst has the lowest horizontal force, and there I γ answers approximately to 0.′2 of declination. Thus when Mr. Nordmann on p. 20 gives values to 0.1 of his unit, he is practically recording declination changes to 0.′003 or less. It is unusual to measure declination curves to nearer than 0.′1 (answering usually to 0.′1 mm approximately), and two observers will often differ by this amount in their estimate. Mr. Nordmann, it is true, has used not his actual curve measurements, but smoothed means of the type ( a + 2 b + c )/4; but even taking this into account one cannot but regard the figures on pp. 20 and 21 as suggesting an accuracy at least ten times that ordinarily attempted.