156 Messrs. Young, G-errard, and Jevons on Electrical 



given together with navigator's observations, e is the earth- 

 induced E.M.F. calculated from the observed tidal velocity 

 on the basis of 2 millivolts per 100 yards per knot. On 

 some occasions simultaneous records were made with two 

 independent pairs of electrodes towed side by side. In such 

 cases both results are tabulated. 



The experimental conditions were not sufficiently satis- 

 factory to warrant any close analysis of the results. The 

 two chief drawbacks were (a) the absence of any proper 

 means of measuring tidal velocities ; (b) the doubtful suit- 

 ability of the Dartmouth waters for initial experiments 

 owing to the uncertain nature of the tides. In some cases 

 the navigator's observations were found to be at variance 

 with the tide-tables, and the discrepancy may have been due 

 to eddy effects or wind disturbances. 



Some idea of the degree of consistency of the electrical 

 measurements may be gained from the bracketed results 

 under the heading "observed E.M.F." It will be seen in 

 such cases that the directions agree to within a point 

 (about 10°) whilst the values of <? 2 agree to within 0*7 milli- 

 volt. 



In regard to direction the table shows a tolerably good 

 general agreement between " estimated " and " observed " 

 directions, only 2 (Nos. 7 & 8) out of the 11 records giving 

 the wrong sense of direction. 



In regard to the values of e 2 , it will be observed that they 

 are of the same order as the corresponding values of e. 

 According to the simple theory e 2 may be less than e, but 

 should not be greater. On the average e 2 is rather less than 

 e and only in one case (No. 3) is it definitely greater. No 

 definite evidence of a constant ratio between e 2 an d e is 

 perceptible even for records made in the same locality. It 

 must, however, be pointed out that the probable errors in e 

 and <? 2 would doubtless mask any such relation if it exists. 



Further records of tidal squares are given in PI. II. figs. 

 8 &9. 



Fig. 8 shows a much steadier zero line on each course 

 than in fig. 7 A. 



Fig. 9 is of interest as showing very clearly the electrical 

 disturbances due to wave-motion. Theory would predict- 

 that the electrical oscillations would occur mainly in 

 a direction transverse to the direction of propagation, so 

 that they would be most effectively registered by the 

 electrodes when the sea is abeam. On the occasion in 

 question the tide and wind were opposed, creating a rather 

 rough beam sea when the vessel was crossing the tide. 



