4 
Sulphates and Chlorides, and that it will form a rich surface 
soil on those parts upon which it has fallen, and that, although 
cattle are suffering from want of food occasioned by the 
burial of the grass, there will be a rich future for the farmer. 
I am indebted to Mr. G. W. Davies, F.C.S., for the above 
analytical report. 
Mr. Henry Wilde, F.KS., exhibited Volcanic Dust from 
Krakatoa, collected during the eruption. 
Mr. W. W. Haldane Gee, B.Sc., exhibited and described 
''An Improved Form of Rheostat.” The instrument, which 
has been made for the Owens College Physical Laboratory, 
consists of a hollow ebonite cylinder, cut with a spiral groove, 
within which 23 feet of No. 18 German silver wire is laid. 
The cylinder is mounted on brass axes to which the ends of 
the German silver wire are connected. Strong brass springs in 
connection with binding screws make contact with these 
axes. One axis is cut with a screw of the same pitch as 
that on the cylinder. The axes are mounted in bearings so 
that when the cylinder is rotated it also has a progressive 
motion. Contact is made with the rheostat wire by a 
platinum wedge fixed to a fine adjusting screw. By pressing 
a lever the platinum, contact is raised above the wire and a 
knife edge is simultaneously withdrawn from the trace of 
the axial screw, allowing the cylinder to be slided bodily, so 
that the platinum contact may be quickly set to any part of 
the rheostat wire. The working parts of the instrument are 
enclosed within a box with glass sides and top. The chief 
mechanical principle of this rheostat is the same as that 
employed in the construction of the rheostats of Jacobi^ and 
Shelford Bidwell,-|* but the details of the improved instrument 
are quite different. 
* Pogg. Ann., 59, page 145. 1843, 
t Phil. Mag., July, 1886. 
