434 



latitudes ; for here, whilst in wooden ships, where the iron work is 

 in detached masses, the ship can have but little, externally, of the 

 character of a true magnet, and can possess but small comparative 

 differences from the position of her head whilst building ; in iron 

 ships, on the converse, where the ship is rendered by percussive 

 action a powerful and, retentively, true magnet, her deviating action 

 must be expected to be different, as the polarity of the head or stern 

 may differ in denomination, or as the ship's magnetic polar axis may 

 happen to lie over to starboard or port. 



As an objection might be made to deductions from experiments on 

 simple individual bars or plates of iron being applied to the case of 

 iron ships built up of thousands of pieces, I have repeated the expe- 

 riments, substituting for an entire plate or bar of iron a plate about 

 18 inches long and 3 broad, made up of numerous separate plates, 

 and combined in the manner of the plating of iron ships. The 

 compound or combined plate of some eighteen or twenty pieces 

 yielded, under percussion, vibration or bending, results precisely 

 similar to those obtained by the use of single plates or bars. 



III. Extract of a Letter from Professor LANGBERG of Christi- 

 ania to Colonel SABINE, dated June 10th, 1855. Com- 

 municated by Colonel SABINE, V.P. and Treas. R.S. 



" Of all the important results from the discussions of the British 

 Colonial Observatory, the discovery of the direct action of the sun 

 on the magnetism of the earth is certainly a fact of the highest in- 

 terest, in opening quite a new field for investigation; and few modern 

 discoveries in this branch of science have interested me more than 

 yours of the annual variation of the diurnal variation of declination. 

 It seems that M. Secchi of Rome has nearly touched at the same 

 discovery, and I am indeed glad that the enormous quantity of cal- 

 culations, which you are superintending, did not prevent you from 

 publishing your results before the ripening fruit was plucked by an- 

 other. The first beautiful result of this annual variation is the ex- 

 planation of the fact, which you have deduced from the observations 

 at St. Helena and the Cape of Good Hope, that the horary variation 



