184 COSMOS. 



ever different therefore the modifications may be which are 

 excited in substances in their molecular state, and in the coer- 

 cive force depending upon that condition in experiments of this 

 nature, there will still remain a considerable thickness of the 

 terrestrial stratum, which might be assumed to be the seat of 

 magnetic currents. The old explanation of the horary varia- 

 tions of declination by the progressive warming of the Earth 

 in the apparent revolution of the Sun from east to west, must 

 be limited to the uppermost surface ; since thermometers sunk 

 into the Earth, which are now being accurately observed at so 

 many different places, show how slowly the solar heat pene- 

 trates even to the inconsiderable depth of a few feet. More- 

 over, the thermic condition of the surface of water by which 

 two-thirds of our planet is covered, is not favourable to such 

 modes of explanation, when we have reference to an immediate 

 action and not to an effect of induction in the aerial and 

 aqueous investment of our terrestrial globe. 



In the present condition of our knowledge it is impossible to 

 afford a satisfactory reply to all questions regarding the ulti- 

 mate physical causes of these phenomena. It is only with refer- 

 ence to that which presents itself in the triple manifestations 

 of the terrestrial force, as a measurable relation of space and 

 time, and as a stable element in the midst of change, that sci- 

 ence has recently made such brilliant advances by the aid of 

 the determination of mean numerical values. From Toronto 

 in Upper Canada, to the Cape of Good Hope and Van Diemen's 

 Land, from. Paris to Pekin, the Earth has been covered, since 

 1828, Avith magnetic observatories,* in which every regular or 



* As the first demand for the establishment of these observatories 

 (a net-work of stations, provided with similar instruments) proceeded 

 from me, I did not dare to cherish the hope that I should live long 

 enough to see the time when both hemispheres should be uniformly 

 covered with magnetic houses under the associated activity of able 

 physicists and astronomers. This has, however, been accomplished, 

 and chiefly through the liberal and continued support of the Russian and 

 British Governments. 



In the years 1806 and 1807, I and my friend and fellow-labourer, 

 Herr Oltmanns, whilst at Berlin, observed the movements of the needle, 

 especially at the times of the solstices and equinoxes, from hour to hour, 

 nnd often from half-hour to half-hour, for five or six days and nights unin- 

 terruptedly. I had persuaded myself that continuous and uninterrupted 

 observations of several days and nights (observatio perpetua) were pre- 

 ferable to the single observations of many months. The apparatus, 



