- 
Direction of the Spark from Secondary Currents. 191 
stead of a slender stype. 
The larger Graptolite marked a on figure 1, seems to have 
belonged to another species, its marking is somewhat indistinct 
under a high magnifying power, but it seems to be covered with 
close set setze much appressed and verticillate in their general 
arrangement. ‘T'’o the naked eye it appears to be longitudinally 
wrinkled. 
Position and locality—Osceola Mills, near the falls of St. 
Croix River, in a thin seam of calcareo-aluminous shale, fifty feet — 
above the water level. 
Arr. XXI.—On the direction of the Spark from Secondary Cur- 
rents under the influence of Helices or Magnets ; by Prof. 
Cuas. G. Pace, M.D., Washington, D. C. 
Iv the course of my experiments, recently communicated, in 
which secondary sparks were obtained from two to six inc es In 
length, the spark seemed to have direction, and in the operation 
of the electro-magnetic engine this peculiar property of the spark 
Was occasionally annoying from the fact of its running along and Z 
oxydating metallic surfaces which were intended to be kept clean _ 
and bright. On investigating this matter, I found the spark to 
be subject to the same tangential or rotative action as the con- 
ductor itself, 
Let 
pole of the battery, 6, a 
block of copper in con- 
made, and ; 
the current then broken, a dense spark or flame, S, about one. 
‘Meh in height is directed along the surface of the block from g 
Oe. The lower line of the spark or that portion on the su 
a, represent one ‘ 
