684 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
whence 
L=2'4028 X 10 s centims. 
The value by d -priori calculation is 
L=2*400Xl0 8 centims. 
about one part in a thousand lo wer. * 
Correction for level. 
If the axis of rotation deviate from the vertical in the plane of the meridian a 
corresponding correction is required. If I be the angle of dip, and /3 the deviation of 
the axis from the vertical towards the north, the electromotive forces are increased in 
the ratio (1+ tan I /3) : 1, in which proportion we must suppose GK increased. (See 
pp. 106, 124 of former paper.) The angle of dip at Greenwich for 1881 is about 
67° 30', so that 
tan I=2‘414 
The correction for an error in level is thus of the first order, and is magnified by 
the largeness of the angle of dip in these latitudes. If the experiments were made at 
the magnetic equator, we should not only reduce the correction for level to the second 
order, but also obtain the advantage of a nearly doubled horizontal force. 
Observations on the level were made by Dr. Schuster on June 1, by myself on 
August 30, and by Mrs. Sedgwick on October 13, and on November 11 and 23. The 
August observations gave f3='26'; the October observations gave /3=‘ 30'; and the 
November observations gave /3=’25' f The position of the axis is necessarily to a 
slight extent indefinite, and the differences are probably accidental. The same level 
was used throughout, and the value of its graduations was tested. We may take 
and 
/3= -\-‘27'= + *000079 circular measure 
1+ tan I /3= 1-00019 
* A further small correction is called for by the fact that at actual temperature of the room (about 
14°) the resistances given by the boxes were not exactly multiples of the B.A. unit. The difference in 
the case of the principal box, which is marked as correct at 14°"2, may be neglected, but the resistances 
taken from the auxiliary box (marked 18°‘3) must have been smaller than their nominal value, to the 
extent of a little over one part in a thousand. By the same fraction £Q, and consequently L, must be 
greater than is supposed in the above calculation. The corrected value of L will be 
L=2‘4052 x 10 8 
It is about two parts in a thousand greater than that found from the measured dimensions, and is, in my 
opinion, quite as likely to be correct. 
