MAGNETIC OBSERVATIONS. 79 



1849. Emory, Magnetic observations made at the Isth- 

 mus of Panama. 



1849. Professor William Thomson, of Glasgow, A Mathe- 

 matical Theory of Magnetism, in the Phil. Transact, for 1851, 

 pt. i., p. 243-285. (On the problem of the distribution of 

 magnetic force, compare 42 and 56, with Poisson, in the 

 Mem. de I'lnstitut., 1811, pt. i., p. 1 ; pt. ii., p. 163.) 



1850. Airy, On the present state and prospects of the 

 science of Terrestrial Magnetism the fragment of what 

 promises to be a most admirable treatise. 



1852. Kreil, Influence of the Moon on Magnetic Declina- 

 tion at Prague in the years 1839-1849. On the earlier la- 

 bors of this accurate observer, between 1836 and 1838, see 

 Osservazioni sulP intensita e sulla direzione della forza magnet- 

 ica instituite negli anni 1836-1838 aW I. R. Osservatorio di 

 Milano, p. 171 ; and also his Magnetical and Meteorological 

 Observation* at Prague, vol. i., p. 59. 



1852. Faraday, On Lines of Magnetic Force, and their 

 definite character. 



1852. Sabine's new proof deduced from observations at 

 Toronto, Hobarton, St. Helena, and the Cape of Good Hope 

 (from 1841 to 1851), that every where between the hours of 

 seven and eight in the morning the magnetic declination ex- 

 hibits an annual period ; in which the northern solstice pre- 

 sents the greatest eastern elongation, and the southern sol- 

 stice the greatest western elongation, without the temperature 

 of the atmosphere or the earth's crust evincing a maximum 

 or minimum at these turning periods. Compare the second 

 volume of the Observations made at Toronto, p. xvii., with 

 the two treatises of Sabine, already referred to, on the Influ- 

 ence of the sun's vicinity (Phil Transact for 1850, pt. i., 

 p. 216), and of the solar spots (Phil. Transact, for 1852, 

 Pt. i-, P- 121). 



The chronological enumeration of the progress of our 

 knowledge of terrestrial magnetism during half a century, 

 which I have uninterruptedly watched with the keenest in- 

 terest, exhibits a successful striving toward the attainment 



ion, is the origin of one part of the variations in the elements of ter- 

 restrial magnetism. Plucker finds that, as the force with which the 

 magnet acts upon the oxygen is proportional to the density of this gas, 

 the magnet presents a simple eudiometric means of recognizing the 

 presence of free oxygen gas in a gaseous mixture even to the 100th or 

 200th part. 



