218 C. A. Perkins — Magnetic Permeability of Nickel. 



in the limestone No. 1, in close proximity to one of the fossil 

 localities and nearly at the same level, occurs a small isolated 

 mass of decomposed gypsum possibly 10 cubic feet in dimen- 

 sions, which is due apparently to the agency of a small sulphur- 

 ous percolation now extinct. 



I believe, therefore, that the structure of the great gypsum 

 deposits of Cayuga Co. separates them sharply from those exist- 

 ing elsewhere in New York in strata of the Salina Period ; and 

 that their association with limestones, both below and above 

 them, containing fossils of the lower divisions of the Lower 

 Helderberg, as well as the nearly uniform character of the rock 

 series from the lake level to the Oriskany Sandstone, vindicates 

 for them a place in the lower portion of the Lower Helderberg 

 in which I include the Water Lime Group. 



A word in conclusion as to the dip of the strata at this point. 

 It will be seen that the top of the Lower Helderberg limestones 

 in the line of section is 110 feet above the lake. About 2f 

 miles south of this point, these limestones disappear beneath 

 the lake, giving a dip of more than 40 feet per mile. This 

 corresponds with some determinations of dip published in this 

 Journal, October, 1883, made on a belt fifty miles long from 

 east to west, and from six to ten miles wide, lying six miles south 

 of Union Springs. I have at present no data for ascertaining 

 whether the effective dip in the intermediate space corresponds 

 with these determinations. 



Art. XXIX. — On the variation of the Magnetic Permeability of 

 Nickel at different temperatures ; by Charles A. Perkins. 



In the year 1820 Oersted published his discovery of the ac- 

 tion of an electric current upon a magnet and Arago discovered 

 that the current would magnetize an iron bar. The same year 

 Ampere proposed his theory of molecular currents to explain 

 both phenomena. Poisson soon after elaborated a theory based 

 on the existence of magnetic fluids, but the tendency has always 

 been toward the acceptance of Ampere's hypothesis and later 

 physicists have striven principally to modify and enlarge it, to 

 make it correspond with the known facts of magnetism. 



It was upon this as a basis, that Weber constructed his theory 

 assuming the existence of molecular currents which an in- 

 ductive force could so arrange as to produce a magnet, while 

 they resisted this tendency to a certain degree. 



With Maxwell's additions this theorj^ gives a rational formula 

 which agrees much better with the known facts than do many 

 of the empirical formulas that have been proposed. 



