184 Mr. C. Marett on the causes of the Intervals 



sulphate of iron, and employed as an experimental core ; the 

 velocity given to the machine at this and all average times 

 was such as to cause five or six approaches and withdrawals of 

 the core in one second; yet the solution produced no sensible 

 indication on the galvanometer/' Referring to Table XXV., it 

 will be seen that the instrument made use of in the present 

 inquiry has given with a solution of protosulphate of iron a de- 

 flection amounting to no less than one hundred divisions of the 

 scale. Mr. Faraday proceeds : — " A tube filled with small cry- 

 stals of protosulphate of iron caused the needle to move about 



2° Red oxide of iron produced the least possible effect.'' 



In the experiments recorded in the foregoing pages, the crystal- 

 lized sulphate of iron gave a deflection of nearly two hundred 

 divisions of the scale, while the red oxide gave a deflection as 

 wide as the helices would permit, which corresponds to about 

 eight hundred divisions of the scale. The correctness of Mr. 

 Faraday's statement regarding the inferiority of the means first 

 devised to investigate this subject, is thus strikingly illustrated. 

 It might be added, that red ferroprussiate of potash and other 

 substances, which have given us powerful effects, produced no 

 sensible impression in experiments made with the other instru- 

 ment. 



Thus have we seen the objections raised against diamagnetic 

 polarity fall away one by one, and a body of evidence accumu- 

 lated in its favour, which places it among the most firmly 

 established truths of science. This I cannot help thinking is 

 mainly to be attributed to the bold and sincere questioning of 

 the principle when it seemed questionable. The cause of science 

 is more truly served, even by the denial of what may be a truth, 

 than by the indolent acceptance of it on insufficient grounds. 

 Such denials drive us to a deeper communion with Nature, and, 

 as in the present instance, compel us through severe and labo- 

 rious inquiry to strive after certainty, instead of resting satisfied, 

 as we are prone to do, with mere probable conjecture. 

 Royal Institution, November 1855. 



XXIV. An attempt to investigate the causes of the Intervals between 

 High-water and Slack-water, and between the turn of Tide near 

 the Shore and in the Offing. By C. Marett, M,A., Trinity 

 College f Cambridge"^. 



IT is well known to all accurate observers on the subject, that 

 in tidal channels and in tidal rivers, the turn of tide, or 

 slack-water, never exactly coincides with high- water, and that the 

 tide near the shore always stops and turns before the tide in mid- 



* Communicated by the Author. 



