Hydrography^ and the Art of Navigation. 29 



posed that this movement is accompanied with a change of form. 

 The study of Hnes of equal inclination, regarded under the same 

 point of view, is attended with equal interest. When all these 

 lines shall have been traced upon the charts, it would be curious 

 to follow them with the eye in all their displacements and changes 

 of curvature ; important truths may emanate from such an ex- 

 amination. It will now be understood why we require as many 

 measurements of inclination as can be collected. 



The question has been often agitated, whether, in a determi- 

 nate place, the inclination of the needle would mark exactly the 

 same degree at the surface of the ground, at a great height in 

 the air, and at a great depth in a mine. The absence of uni- 

 formity in the chemical composition of the earth, renders the 

 solution of this problem very difficult. If observations are 

 made in a balloon, the measurements are not sufficiently exact. 

 When the observer takes his station on a mountain, he is ex- 

 posed to local attractions ; ferruginous masses may then greatly 

 alter the position of the needle, without there being any thing to 

 make him aware of the fact. The same uncertainty affects ob- 

 servations made in the galleries of mines. Not that it is abso- 

 lutely impossible to determine the influence of accidental circum- 

 stances in each place ; but for that purpose it is necessary to 

 have instruments of the most perfect kind ; it is necessary to 

 be able to go to a distance, and in all directions, from the station 

 which one has chosen ; and, finally, to repeat the experiments much 

 more frequently than a traveller generally has an opportunity 

 of doing. But, however this may be, observations of this kind 

 are worthy of attention. Viewing the whole of them in connec- 

 tion» they will perhaps one day lead to some general result. 



With regard to the declination, its immense utility is so much 

 experienced by navigators, that any recommendation on the sub- 

 ject would be superfluous. 



Observations on Intensity. — Observations on intensity are not 

 of earlier date than the travels of Entrecasteaux and M. de 

 Humboldt, and yet they have already thrown a bright light on 

 the complicated, but at the same time highly interesting subject, 

 of terrestrial magnetism. Observations of this nature ought, in 

 the highest degree, to attract the attention of the officers of the 



