84 Observations upon the Dip of the Magnetic Needle. 



From the above facts, some important hints might be afforded 

 to analytical chemistry, for it will be at once seen that the pres- 

 ence of any of the alkaline salts in a solution from which it 

 might be wished to precipitate lead in the form of a sulphate, 

 would affect the accuracy of the result. What is true of the 

 sulphate of lead, may be found also true for other insoluble salts. 

 Moreover, this shows the importance, in the analysis of mineral 

 waters for instance, of weighing well the relative strength of the 

 various acids and bases therein found, in order to ascertain what 

 salts are present, and not to be contented with evaporating the 

 water to dryness, and considering such salts as remain to be those 

 existing in the water, for many of them may be formed during 

 the evaporation. It is not at all improbable, that before many 

 years the examination of mineral waters will be based as much 

 upon calculation as upon analysis, the former of course being 

 guided by the latter, and by certain laws not yet developed. 



Art. VI. — Observations upon the Dip of the Magnetic Needle; 

 by Thomas Hobart Perry, Prof. Mathematics, U. S. Navy. 



The instrument used in obtaining the following results, was 

 made at the establishment of Messrs. E. & G, W. Blunt, of New 

 York, in the autumn of 1837. The needles are about five and a 

 half inches in length, and the vertical circle is six inches in diam- 

 eter, and the horizontal seven ; each being divided to 20' of a 

 degree, and the latter reading by a vernier to 20''. The needle 

 and vertical or dip circle are protected by a cylindrical brass case 

 with glass bases, and the whole is arranged very much in the 

 same way as one of Dollond's, with the exception that the axes 

 rest in agate knife-edge Ys ; a contrivance which though less 

 eligible because less delicate than planes, has nevertheless the 

 advantage of ensuring to the pivots very nearly the same position 

 after each reversion ; and, as at sea the motion of the ship does 

 not allow the needle to be entirely at rest, and on land care is 

 taken to tap the case gently before each reading, and no observa- 

 tion is recorded without the proper reversions of the circles, axes, 

 and magnetism, — it is hoped that the errors resulting from the 

 form of the Ys have, with others, been eliminated. 



