9-t Mr. J. Croll on the Physical Cause of Ocean-currents. 



gate resistances, the resistance of the latter does not appear in 

 the expression for the resistance external to the galvanometer, 

 which is not in any way affected by the battery resistance. Si- 

 milarly for the resistance external to the battery, which, at a 

 balance, is independent of the galvanometer resistance. I cannot 

 agree with Mr. B rough that to find the resistance in either case 

 at a balance is a mere mathematical problem destitute of phy- 

 sical meaning ; for it is only when at a balance that the problem 

 has any practical importance. 



As Mr. B rough most truly observes, most Wheatstone's bridges 

 are wrongly arranged. An excellent example of this once came 

 under my notice. A gentleman informed me he was about to 

 make a Wheatstone's bridge, a great improvement, and very eco- 

 nomical. Instead of using three separate sets of resistance-coils 

 {a, by c) he would use only one (c) \ for he would make a and b 

 equal to 0. Mr. Harris's arrangement appeared to succeed ad- 

 mirably. There was no difficulty whatever in getting a zero; in 

 fact there was always a balance, whether the line under examina- 

 tion was long or short. There was only one drawback ; and that 

 was, the improvement afforded no information whatever as to 

 the resistance of the line. 



I am &c, 



Oliver Heaviside. 



P.S. — The condition that the galvanometer should connect the 

 junction of the two greatest with the junction of the two least of 

 the resistances, is necessarily complied with by the equations I 

 have given for the best arrangement with a given galvanometer 

 and battery ; else it would not be the best arrangement. 



XIV. On Ocean- currents. — Part III. On the Physical Cause oj 

 Ocean-currents. By James Croll, of the Geological Survey 

 of Scotland. 



[Continued from vol. xlii. p. 280.] 



Further Examination of the Gravitation Theory of Oceanic Circu- 

 lation. 



Introduction. 



EEW subjects have excited more interest and attention than 

 the cause of ocean circulation ; and yet few are in a more 

 imperfect and unsatisfactory condition, nor is there any' ques- 

 tion regarding which a greater diversity of opinion has prevailed. 

 Our incomplete acquaintance with the facts relating to the cur- 

 rents of the ocean and the modes of circulation actually in ope- 

 ration, is no doubt one reason for this state of things. But 

 doubtless the principal cause of such diversity of opinion lies in 



