Oil a supposed Periodicity in Terrestrial Magnetism. 141 



was perceptible on the copper. On continuing this experiment for 

 three liours, it was found that the copper plate lost 0*003 grm., and 

 the silver plate was increased by 0*004 grm. On cleaning the plates, 

 and reversing their position, the copper was covered with a film of 

 oxide, while the silver remained free from cupreous deposit. We 

 believe therefore that, through the simultaneous action of the two 

 metals, the dissolved salt is put into such a state of tension that 

 oxygen brings about a chemical change which otherwise would be 

 impossible, and that this change is initiated in close proximity to the 

 more negative metal. 



Though we have examined only this particular reaction, we have 

 satisfied ourselves that it is not an isolated fact. Each of the ele- 

 ments concerned may be replaced by others : thus the sulphate may 

 be substituted for the nitrate of copper, or platinum may be used 

 instead of silver ; chlorine may take the place of oxygen, with the 

 production of the subchloride instead of the suboxide ; and zinc may 

 be employed as the positive metal, with zinc chloride as the salt in 

 solution, in which case copper may be taken as the negative metal, 

 and on its surface will form a deposit of oxide of zinc. 



April 25. — George Biddell Airy, C.B., President, in the Chair. 



The following communication was read : — 



"On a supposed Periodicity in the elements of Terrestrial Mag- 

 netism, with a period of 26^ days." By George Biddell Airy, Astro- 

 nomer Royal. 



In a paper published in the * Proceedings of the Imperial Aca- 

 demy of Sciences of Vienna,' vol. Ixiv., Dr. Karl Hornstein has exhi- 

 bited the results of a series of observations which appeared to show 

 that the earth's magnetism undergoes a periodical change in suc- 

 cessive periods of 26^ days, which might with great plausibility be 

 referred to the rotation of the sun. 



It appeared to me that the deductions from the magnetic obser- 

 vations made at the Royal Observatory of Greenwich, and which 

 are printed annually in the ' Greenwich Observations,' or in the 

 detached copies of ' Results of Magnetical and Meteorological Ob- 

 servations made at the Royal Observatory of Greenwich,' would 

 aiford good m.aterials for testing the accuracy of this law, as appli- 

 cable to a series of years. The mean results of the measured hourly 

 ordinates of the terrestrial magnetic elem.ents are given for every 

 day ; and it is certain that there has been no change of adjustments 

 of the declination and horizontal-force instruments in the course of 

 each year. For the horizontal-force instrument the temperature of 

 the room has been maintained in a generally equable state, and in 

 later years it has been remarkably uniform. 



It is easy to see that an error of a single day, or of a large fraction 

 of a day, in the beginning of each period, is of no importance, pro- 

 vided that the errors are not permitted to accumulate. It was allow- 

 able, therefore, to take successive periods of 26, 2G, 27, 26, 26, 27, 

 &c. days ; and in instances when a single day was omitted, or eveu 



