Polytechnic Association. 729 



magnetism of the earth. But in the explosion seen by Professor 

 Young, it was a movement of very light matter, and it is not 

 surprising that the effect should have been so slight as to be 

 imperceptible by means of the magnetometer. The most plausi- 

 ble explanation of the cause, the magnetism of the earth, is that it is 

 due to thermo-electricity produced by the difference of the action of 

 solar heat on different parts of its surface. I asked Prof. Meyer last 

 night, after his lecture before the Institute, on magnetism, why he did 

 not speak of the cause of the earth's magnetism. He replied that 

 although the thermo-electric theory is very plausible, he was not pre- 

 pared to indorse it positively ; but he aimed simply to show the fact 

 that the earth is a magnet. We had in that lecture one experiment 

 which was never before made in the world ; the minutest change of 

 the galvanometer was visible to the whole audience. 



Dr. Van der "Weyde — A great many experiments in magnetism 

 have heretofore been such that they could be seen by a dozen persons 

 at most. But the use of the vertical lantern allows a whole audience 

 to see the results of such experiments. The nearest approach to this 

 plan was that which I have tried, throwing the image upon the ceil- 

 ing : but Prof. Morton's improvement of Prof. Cooke's plan is far 

 superior. The experiments themselves, which were exhibited last 

 evening, were not new ; but they have never before been exhibited to 

 a large audience. 



The first person to announce the theory that the earth's magnetism - 

 is caused by the solar heat was Barlow. But there are two objections 

 to that theory. One is that the action of a sun-spot is so imrriediate. 

 If it is the result of currents, it should take some time. It takes three 

 hours to get the maximum of heat ; it does not occur when the sun 

 passes the meridian. The velocity of the transmission of this influ- 

 ence is very great. The velocity of light is slow in comparison to it. 

 There is positive proof that the transmission of gravitation is instan- 

 taneous, proof as certain as that the three angles of a plane triangle 

 are equal to two right angles. It is very likely that the transmission 

 of magnetism from the sun to the earth is instantaneous. The second 

 objection is that it does not explain the rotation of the magnetic pole 

 around the geographic pole. There are two north poles; but the 

 principal one is in this country now. Two hundred years ago it was in 

 Europe; and it is now going back toward Europe. The cause of this 

 is unknown. It may be that the earth is magnetic by induction 

 chiefly. 



The President — I do not accept the force of the first objection; for 



