NATURAL PHILOSOPHY. 135 



of the daily variation of temperature, and the air thus heated and 

 cooled affects the lines of magnetic force which pass through it in 

 their course from the earth into space. As the sun rises and comes 

 onward in relation to any given place, the atmosphere heneath is af- 

 fected, so as to cause the lines of magnetic force to diverge within the 

 heated mass, and as the sun passes away, and air of lower temperature 

 than the mean is produced, the lines of force tend to converge. It is 

 not assumed that the hot or cold air acts at once upon the needle, but 

 upon the great system of magnetic forces, which, emanating from the 

 earth, pass, as it were, through the atmosphere into the regions of 

 space, and, because of their polarity, return to the earth again. When 

 these are affected in any one part, needles everywhere upon the sur- 

 face of the planet are also affected in proportion to their distance from 

 the seat of action, following in their position the force of the curve 

 which governs them. Upon a first comparison of the alteration which 

 should occur in the lines of force at any given place for a given hour 

 with the magnetic observations made at that place, the directions of 

 the variations, both of the declination and inclination, are found to 

 accord so well with the theoretical deductions, as to create a strong 

 expectation that the assigned cause is the true physical cause of the 

 annual and diurnal variations, and of many of the irregular variations 

 that are made evident in the records of the magnetic observatories. 



MAGNETO-OPTIC PROPERTIES OF CRYSTALS. 



MESSRS. TYNDALL AND KNOBLAUCH presented to the British Asso- 

 ciation, at Edinburgh, a paper on "the magneto-optic properties of 

 crystals, and the relation of magnetism and diamagnetism to molecular 

 arrangement." Their conclusions, derived from the examination of 

 more than a hundred natural crystals, are thus summed up : " We 

 have on the one side four new forces assumed, the optic attractive and 

 the optic repulsive forces, the maguo-crystallic and the magneto-crys- 

 tallic forces ; and on the other side no new force whatever, but simply 

 that modification of existing forces which we have named electro- 

 polarity, By attention to the compression of amorphous bodies, every 

 single experiment cited can be reproduced." London Athenaum, 

 Aug. 



POWERFUL MAGNETS. 



SIR DAVID BREWSTER exhibited to the British Association two 

 powerful magnets made by a process devised by M. Elias, and manu- 

 factured by M. Logeman, at Haerlem. One of them, a single horse- 

 shoe magnet, weighing about lib., is capable of lifting SS^lbs., and 

 the other, a triple horse-shoe magnet of about 10 Ibs. weight, is capa- 

 ble of lifting about 150 Ibs. Similar magnets are made by the same 

 person, capable of supporting 5 cwt. They are made by some peculiar 

 process, in which a helix of copper and a galvanic battery are used ; 

 and are so permanent that they suffer little, if at all, from having their 

 soft iron guards forced off several times abruptly. They are accom- 



