SCIENTIFIC SUMMARY. 
431 
feebled or destroyed. The presence of the gas is an important element to 
be considered, as by varying it the current may be intercepted or the appear* 
ance of the light greatly altered. Similar effects are attained by varying 
the strength of the current, and are most marked when the current is feeble. 
The phenomena are the same whether a Holtz machine or coil are used as 
a source of electricity, and are independent of the direction through the coil 
or gas. The form of the armatures is important ; they should be plain and 
surround the tube for the greater part of its length. The effect of a single 
pole is slight, or of both poles when more than five millimetres distant from 
the tube. 
Different Forms of Light for Lighthouses. — At the recent meeting of the 
British Association Sir W. Thomson and Mr. J. Hopkinson read a paper on 
tl Methods for giving Distinctive Characters to Lighthouses.” Sir William 
Thomson read the first portion. Speaking of coloured lights, he said they 
would not be of value except for marking a specific direction, and for this 
colour had been the only successful invention. At Ardrossan a ship went 
ashore through a mistake of a light in Ardrossan for a harbour light. There 
was a red light in an apothecary’s shop in Ardrossan, and the pilots had 
told him that they regularly steered in by the u light of the doctor’s shop.” 
The greater speed of steam traffic required that light should be seen at a 
greater distance and recognised sooner, and the lights must be more 
powerful. Rapid advances have been made in the English lights, particu- 
larly in respect to their power, but more distinctions were required. Many 
harbour lights were now confounded with gas. Now there was a blaze of 
gas, and it was, in some cases, impossible to make out which was which. 
The authorities -were exceedingly sluggish in making such changes as were 
required in the appliances. The eclipse light, which he advocated, would 
signal three lights, which he described as “ short, short, long,” indicating 
the periods for which the light would be eclipsed. Mr. Hopkinson had also 
invented a revolving light, which cost little more than the ordinary revolv- 
ing apparatus, but which would give a double flash or a treble flash, instead 
of the one flash of the ordinary revolving apparatus. 
Effects of Stress on the Magnetism of Soft Iron. — Sir W. Thomson read a 
paper at the meeting of the British Association, in continuation of his essays 
before the Royal Society. In the physical laboratory at Glasgow University 
he had stretched steel and soft iron wire about twenty feet long from the 
roof. An electro-magnetic helix was placed round a few inches of the wire, 
so that the latter could be magnetised when an electric current was passed 
through the former ; the induced current thus produced in a second helix 
outside the first being indicated by a reflecting galvanometer. When steel 
wire was used, the magnetism diminished when weights were attached to 
the wire, and increased when they were taken off ; but when specially made 
soft iron wire (wire almost as soft as lead), the magnetism was increased 
when weights were put on, and diminished when they were taken off. 
Afterwards he discarded the electrical apparatus, and by suspending a piece 
of soft iron wire near a magnetometer consisting of a needle a small fraction 
of a grain in weight, with a reflecting mirror attached, the wire was magne- 
tised inductively simply by the magnetism of the earth, and changes in its 
magnetism were made by applying weights and strains, the changes being 
then indicated by the magnetometer. 
