MAGNETIC OBSERVATIONS. 79 



1849. Emory, Magn. observations made at the Isthmus of 

 Panama. 



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

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

 pt. i. pp. 243 285 (On the problem of the distribution of 

 magnetic force, compare 42 and 56 with Poisson in the 

 Mem de V Institut., 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 pro- 

 mises to be a most admirable treatise. 



1852. Kreil, Influence of the Moon on Magnetic Declination 

 at Prague in the years 1839 1849. On the earlier labours 

 of this accurate observer, between 1836 and 1838, see Osser- 

 vazioni sulV intensita e sulla direzione della forza magnetica 

 institute negli anni 1836 1838 alV I. E. Osservatorio di 

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

 Observations 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 everywhere between the hours of 

 seven and eight in the morning the magnetic declination 

 exhibits an annual period ; in which the northern solstice 

 presents the greatest eastern elongation, and the southern 



finally, in connection with the latter, they further act upon a magnetic 

 needle, which may be assumed to be in or beyond the atmosphere. 

 The envelope of oxygen that encircles our terrestrial sphere may be com- 

 pared to an armature of soft iron upon a natural magnet or a piece of 

 magnetised steel; the magnet may further be assumed to be spherical, 

 like the earth, while the armature is assumed to be a hollow shell, 

 similar to the investment of atmospheric oxygen. The magnetic power 

 which each particle of oxygen may acquire by the constant force of 

 the earth, diminishes with the temperature and the rarefaction of the 

 oxygen gas. When a constant alteration of temperature and an expan- 

 sion follows the sun around the earth from east to west, it must pro- 

 portionally alter the results of the magnetic force of the earth, and of 

 the oxygen investment, and this, according to Faraday's opinion, is the 

 origin of one part of the variations in the elements of terrestrial mag- 

 netism. Plucker finds that as the force with which the m signet acts 

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

 presents a simple eudiometric means of recognising the presence of 

 free oxygen gas iu a gaseous mixture even to the 100th or 200th 

 part. 



