in different Latitudes. 



101 



move it further than the evening before, it follows that the va- 

 riation is increasing from one day to the next ; if the contrary, 

 it is decreasing. The greatest diurnal variations generally take 

 place during the months of April, May, June, and July; that is 

 to say, between the two equinoxes of spring and autumn. They 

 are at Paris from 13 to 16 minutes. The least are from 8 to 

 10 minutes, and they take place during the remainder of the 

 year. Now, if we compare the analogous positions of the needle 

 on different days, but at the same hours, in order to have its 

 general progress, we find that, from the spring equinox to the 

 following summer solstice, the variation is decreasing ; and that 

 it is increasing all the rest of the year, that is, from the sum- 

 mer solstice to the foUov/ing spring equinox (as is represented 

 in fig. 4). For our knowledge of these periods we are indebted 

 to M. Cassini, who has established theu;j by observations made 

 during eight years at the Observatory at Paris. 



Lastly; numerous observations prove that the magnetic needle 

 is subject to sudden and accidental alterations, which depend on 

 the appearance of those luminous meteors called aurorce ho- 



piiical Transactions, vol. 11. p. 899, from whose observations it appears 

 t)iat,aIthoui:li there be great iiretrularities in this diurnal change of position 

 ot the needle, yet there is a certain average whicli is kept up with consi- 

 derable steadiness. The following table shows the averai^e of greatest 

 daily change of position in the different months of the year, observed in 

 I\Jr. Canton's house in Spital Square, London, in 1759. 



Mr. Canton attempts to account for these changes by observing that 

 the force of a magnet is weakened by heat. A small magnet being placed 

 near a compass needle, and E.N.E. from it, so as to make it deflect 45' 

 from the natural position, the magnet was covered with a brass vessel, into 

 whicli hot water was poured. Tlie needle gradually receded from the 

 ina;;net 3--ltiis of a degree, and returned gradually to its place, as the water 

 cooled. 'J iio parts of the earth to the eastward are (list heated in the 

 morning, and tiierefore the magnetic force of tlie earth is weakened, and 

 the needle is made to move to the westward: but as the sun warms the 

 western side of the earth in the afternoon, the needle must move in a con- 

 trary direction. TEpinu.s supjjoses that the sun acts on the earth as a mag- 

 net acts on a piece of soft iron; and in the morning propels the lluid into 

 the N.W. parts. The needle directs itself to this constipated fluid, and 

 therefore it points to the eastward of the magnetic north in the afternoon. 

 Neither of the se cxplanati(ms will account for the great diversity of tiie 

 (linrnal variations in dillerent places, which are so great, tliat Professor 

 Ilubison thinks, wc can hardly ascribe the diurnal variation to any change 

 in the magnetism of the primitive terrestrial magnet, and must rather 

 look for its cause in local circumstances. 



G 3 realei. 



