436 



Mr. R. Sliida. 



experiments the value of Jc was so very small and the magnetising 

 force at any point of the wire or bar was so very slightly different 

 from that at the centre, that the error which would arise from using 

 the equation (8) will be very insignificant, and consequently this 

 approximate equation was always used to evaluate F. 



The current strength c was always measured on a Thomson tangent 

 galvanometer, G, except when it was so weak that a small error in 

 the galvanometer reading will produce a considerable error in the 

 result, in which case the current was estimated from the electromotive 

 force of the battery and the resistance of this circuit. 



The strength, S, of the field was calculated in terms of H, the 

 horizontal component of the terrestrial magnetism, simply by comparing 

 the deflections of the magnetometer needle acted upon by a magnet 

 (placed behind and at a convenient distance from the needle, and 

 with its length in the line at right angles to the plane of the magnetic 

 meridian) in the two cases : (1) When the field was due to the 

 horizontal component H alone ; and (2) , when it was due to both the 

 controlling magnet N and the horizontal force H. Since evidently the 

 value of H seriously affects the results, it was thought desirable to 

 make a fresh experiment to determine H at the very spot where the 

 magnetometer needle is suspended. This was effected indirectly by 

 counting the periods of a* magnetic needle at the point in question, 

 and at another point where the exact value of H was known from a 

 direct experimental determination made after the manner described in 

 my paper on " The Number of Electrostatic Units in the Electro- 

 magnetic Unit" (" Phil. Mag." for December, 1880), or more fully 

 explained in Mr. Thomas Gray's paper on " The Experimental 

 Determination of Magnetic Moments in Absolute Measure " (" Phil. 

 Mag." for November, 1878) ; the value of H at the point where 

 the magnetometer hangs was found to be '1590. The value of V, 

 the earth's vertical force, is of by far the less moment, considering 

 that the only results the accuracy of which depends greatly upon that 

 of the value of V, are those for fx for that particular magnetising force 

 only; so that it was deemed nnnecessary to find Y by a new 

 experiment, and consequently it was deduced from the value of H and 

 that of the dip, 73° 45' being taken for the latter according to the 

 determination made some three years ago. 



The final results tabulated at the end of the paper, namely, in the 

 Tables A, B, C, D, &c, were derived from the mathematical con- 

 siderations above discussed, and from the results given in the 

 corresponding Tables of Deflection I, II, III, IY, &c, with the 

 exception of the results given in 7, 8, 9, and 10 of the Table E, and 2, 

 3, and 4 of the Table F. The intensity, I, in these exceptional cases 

 was obtained by assuming that the deflections of the magnetometer 

 needle due to the magnetism of the bar alone (that is to say, the 



