Photography at Greenwich Observatory. 15 



amount of the diurnal variation is greater than in the winter 

 half of the year. In southern latitudes the variation is westerly 

 in the morning and easterly in the afternoon, but it follows 

 the law of local time, and the annual inequality is also coinci- 

 dent with ours, although it is winter there ; and the same law 

 prevailing in the tropics, where the temperature varies but 

 little, proves that, although produced by the sun's influence, 

 it is not an effect of heat. 



There are also sudden irregularities known as magnetic 

 storms, which produce perturbations of greater or less magni- 

 tude simultaneously over the globe, and which have occasionally 

 been found to be contemporaneous with the exhibition of 

 brilliant aurorEe on the earth, or sudden alterations of the 

 surface of the solar disc. The labours of General Sabine have 

 made us acquainted with the fact that these perturbations 

 have the same periods for their maxima and minima as the 

 number of solar spots as determined by Schwabe, both the 

 classes of phenomena having a cycle of about ten years and a 

 quarter, and this solar spot period also influences the whole of 

 the magnetic elements. 



We are indebted to the researches of the late Professor 

 Gauss, of Gottingen, for the foundation of our present position 

 in magnetic science. He, co-operating with the celebrated 

 Humboldt about thirty-five years ago, formed an association 

 for simultaneous observation at various continental stations, 

 and himself devised the methods of observing and the instru- 

 ments still used. He showed that the position of the needle 

 at any time is the result of three forces acting together on the 

 magnet, and that these three forces are the declination, the 

 inclination, and the intensity of the force. The first of these 

 elements is observed by the declination magnetometer, but 

 the two last are not observed directly, but deduced from the 

 variations of the two components, the vertical force and the 

 horizontal force, which conjointly produce the direction of 

 the magnet. The instruments used for this purpose are 

 the dip or balance magnetometer, and the bifilar magneto- 

 meter. 



An ordinary magnet will in most parts of the Northern 

 Hemisphere have the marked end depressed, and as we proceed 

 northward this depression increases, until we reach points 

 where the needle becomes vertical. Thus, in 1831, Sir James 

 Clarke Eoss, in latitude 70° 5' 17" N. and longitude 

 96° 45' 48" W., found the dip to be 89° 59', within one minute 

 of the vertical ; and as in the Southern Hemisphere the opposite 

 end preponderates, the same observer in 1841 found an incli- 

 nation of 88° 36' in latitude 75° 22' S. and longitude 

 161° 48' E. These points are, it will be seen, by no means 



