Local Attraction upon the Magnetic Needle. 23 



continues to be used by land surveyors, with very few excep- 

 tions throughout the whole of this country ; and as it must be 

 evident, that any survey made by the needle in such a district as 

 the one in question must be incorrect, I conceived it would not 

 be unacceptable to many to have such information inserted in 

 a county map. From a mean of ten observations of the sun, I 

 found the variation of the needle from the true meridian to be 

 28° 9' 41" at the point C towards the west; at the point 9, a 

 distance of 45 perches from the former, it was 29° 2', and at 

 8, distant 38 perches from the last, it was 29° 40' ; at the point, 

 7, a distance of 20 perches, it was 30° 4' ; at the point 6, a dis- 

 tance of 15 perches, it was 31° 40'; at the point 5, a distance 

 of 28 perches, it was 32° 45'; at 4, it was 31° 37', the distance 

 41 perches ; at 3, it was 30° 1', the distance 20 perches ; at 2, it 

 was 29° 7', the distance 29 perches ; and at the point 1, it was 

 28° 10'; at which it has ag^ain become very nearly the true 

 annual variation of the year : the lines D, 1,2, 3, &c., are the 

 magnetic meridian, and the red lines shew the deviation of the 

 needle at those points. I have taken the liberty of forwarding 

 you the above trifling remarks, which perhaps you may think 

 worthy of some notice in your valuable and useful publication. 

 I have the honour to be, Sir, 



With respect, your obedient servant, 



John Macneill. 



Art. V. Lamarck's Genera of Shells. 

 (Continued from Vol. XIV. p. 322.) 

 9th Family. 

 Naiada*. (4 Genera.) 

 Fresh water shells. Hinge sometimes with an irregular, 

 simple, or divided cardinal tooth, and a longitudinal tooth ex- 

 tending under the corselet ; sometimes no tooth, or is furnished, 

 through its whole length with irregular, granular tubercles. 

 Muscular impression posterior, compound. Beaks decorticate, 

 often eroded. 



* River Nymphs. 



