At Lockport, is found a coralline, probably a Porites, 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 Porites 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. Some 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 ? 
Arr. IIl—On the Manipulations of the Dipping Compass ; by 
Joun 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 
