50 Dr. G. E. Allan on 
two points towards the ends of the furnace the temperature 
was at a maximum, the extreme difference being about 10° 
for temperatures in the neighbourhood of 400°C., the dif- 
ference being smaller for higher temperatures. The basalt 
bars were of such a length as to lie within the two points 
at maximum temperature, and the actual temperature- 
difference between the ends of the bar and the middle 
probably did not exceed 6° or 7°. 
In employing the magnetometric method it was, of course, 
necessary to reduce all magnetic disturbances to a minimum. 
These were found to be caused chiefly by the two cireuits 
which supplied currents to the magnetizing-coil and the 
furnace from a distant battery of accumulators, and the small 
compensating-coils C, and C3 were included in the two cir- 
cuits and adjusted to neutralize the disturbances mentioned. 
It was also found impossible to carry out extended magnetic 
observations except on Sundays or during the night. <A 
4-minute electric tram service passing at a distance of over 
300 yards did not cause any serious inconvenience. 
To test the apparatus and thermometer together, a small 
strip of mild transformer steel, cbtained from Dr. D. K. 
Morris, the critical temperature of which was known to 
lie between 770° and 780°, was placed in the furnace with the 
thermometer and heated up. The temperature of the critical 
point was found to be 776°C. The apparatus was therefore 
considered to give satisfactory results, the difficulty of course 
remaining, however, with regard to the difference of tempe- 
rature that must in all probability exist between the outside 
and inside of a bar with such low heat-conducting power as 
that of basalt, when it is being heated or cooled. 
Before testing the effect of temperature on basalt, some of 
the bars were passed through cycles of magnetization. A 
4-coil manganin rheostat with mercury cups was made, the 
resistances of the coils being such that currents of about 
0:25, 0:5, 0°75, and 1 ampere could be obtained by putting 
the coils in series, by steps. The current was measured by a 
Weston ammeter of the type called the Century Standard 
Testing Set, which gave 100 divisions per ampere. 
The three curves given, viz. for Rowley Blue No. 2, Rowley 
Dolerite No. 2, and Dattenberg No. 1, show that hysteresis 
exists in these bars, but only in a very limited degree ; and 
curves Nos. I. and II. (Pl. 1.) form an illustration of the point 
emphasised by Prof. Sir A. Riicker in the paper mentioned, 
“that powerful permanent magnetization affords no proof 
of high permeability.” The dolerite when first obtained 
and tested had the greatest effect on a long compass-needle, 
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