554 Proceedings of the Royal Society 
ing the same. I have tried in this way, besides bismuth, cylinders 
of lead, tin, iron, antimony, and carbon, and find that bismuth gives 
by far the best result. In the other cases the sound from the simple 
loose contact is heard clearly enough; but there is hardly any 
increase of it produced by rotating the cylinder. How this seems 
to be due in great part, if not entirely, to the difference between the 
metals as regards their specific heat and thermal conductivity. 
Obviously, with the same current, the greatest heat will be produced 
at the junction of the rubbing point and cylinder, when the specific 
heat and thermal conductivity are both as low as possible. Hence 
very probably the reason why bismuth answers so well, seeing that 
of all common metals it stands lowest on the list both in specific 
heat and thermal conductivity, In fact, if we take the product of 
the reciprocals of the specific heats and thermal conductivities of the 
above-mentioned metals, we find the product for bismuth greatly in 
excess of that for any of the others. 
3. Note on the Present Outbreak of Solar Spots. By 
Professor Piazzi Smyth. 
4 th April 1880. 
The physical activity going on in the Sun is still increasing, and 
worthy of all admiration. There was a very large spot had come 
round on the Horth-f olio wing limb on March 29 ; and was after 
that the subject of observation from day to day as it approached the 
central Solar meridian. But when it arrived there, on April 3, 
behold two less large but still most notable spots had burst out 
clear and full within the previous twenty-four hours between the 
great spot and the preceding limb. And on this day, April 4, there 
are two more notable ones very close to the greatest spot, making in 
all five remarkable spots not only all visible at once, but working and 
seething positively before our eyes. 
April 5, Hoon. Three of yesterday’s five spots are gone. Faculae 
are in their place ; and that is “ the end of spot-life,” says Prof. 
Alex. S. Herschel. 
