1 5 o SCIENTIFIC APPARATUS. 



tions of the magnetic properties of the earth, they gained 

 immensely in interest. Early in the seventeenth century, the 

 variation of magnetic dip with change of geographical position 

 had been discovered by the Jesuit Nicholas Caboeus, who found 

 it to be about 62, in north latitude 45?, and as much as 72 in 

 London. In 1617 he intrusted a carefully made dipping-needle 

 to one of the missionaries of his Order who was about to proceed 

 to China, and although the missionary died upon the journey, 

 Cabceus learned that the dip had continually decreased as the 

 equator was approached, and, that farther south, as far as the 

 Cape of Good Hope, a greater and greater southerly dip had been 

 observed. The secular variations of dip and declination cannot 

 have escaped notice when trustworthy observations of these 

 elements had been accumulated during a considerable number of 

 years. The daily change of declination was detected in 1683 by 

 lachard, a Jesuit missionary, in Siam, and the hourly variation of 

 the same element was observed in March, 1722, by the English 

 instrument maker Graham.* 



By the end of the first quarter of the present century considerable 

 progress had been made in the construction of accurate instruments 

 for magnetic observations, as well as in the methods of using 

 them, and a large number of careful measurements had been 

 accumulated; but the year 1833 marks the beginning of a new 

 era in the history of terrestrial magnetism. In that year Gauss 

 published his celebrated treatise, Intensitas vis magnetic^ terrestris 

 ad mensuram absolutam revocata, which not only at once placed 

 a great part both of the theory and practice of magnetic observa- 

 tion on a new basis, but has been the starting-point of the whole 

 system of " absolute measurement," the importance of which is 

 daily getting to be more fully recognised in all departments of 

 physics. Before Gauss's time, many measurements had been 

 made, particularly by D'Entrecasteaux and Humboldt, and by 



* The historical statements in this paragraph are chiefly taken from 

 Horner's Article " Magnetismus " in Gehler's Physikal. Worterbuch. 



