Manipulations of the Dipping Compass. 235 



At Lockport, is found a coralline, probably a Pontes^ which 

 for distinction's sake, I will call P. gypsea. It occurs alone, or 

 with laminated gypsum, often clear and fine selenite, diffused 

 through it ; or rather, the petrifaction of carbonate of lime has 

 been converted into the sulphate, and crystallized where it was 

 formed. Sometimes this Pontes is divided, as if by a saw^ into 

 separate portions, straight ; and the sections at various inclina- 

 tions to each other. The cuttings are narrow, sometimes a fourth 

 of an inch wide. The gypsum still lies in many of these appa- 

 rent straight cuttings or saw-cuts ; in others the gypsum has been 

 dissolved, and the cuts are empty. S-ome of the specimens have 

 great beauty. It is evident that the petrifaction has undergone 

 this operation. How should it be divided by these straight cuts ? 

 Could the sulphuric acid be generated in that place, and for any 

 reason follow a course which appears to have been drawn by a 

 rule ? 



Art. III. — On the Manipulations of the Dipping Compass ; by 

 John Locke, M. D., Prof, of Chem. in the Med. Coll. of Ohio, 

 and Lecturer on Nat. Philos. in the Ohio Mechanics' Insti- 

 tute. 



Messrs. Silliman — Every practical magnetician is aware of 

 the great difficulty in obtaining consistent results, with even the 

 most improved dipping apparatus. In my late communications 

 to your Journal, it appears that I have succeeded, with an appa- 

 ratus made by Robinson of London, so far that the discrepancies 

 between the results of the two needles used, seldom amount to 

 one minute of a degree. In the same communications I alluded 

 to some peculiar manipulations which I adopted first at Davenport 

 in Iowa, in September of 1839, and to which I attributed the su- 

 perior consistency of my subsequent results. Having been hon- 

 ored with a verbal request by several distinguished collaborators, 

 that I would communicate my views on this subject, I am indu- 

 ced to lay before your readers the following remarks. 



In the instrument used by me, the dipping needle moves on 

 small pivots supported by straight polished agate edges, on which 

 they roll like a wheel upon a rail. As such a motion is not only 

 rotary but progressive, the centre of the needle is carried out of 



