272 
advise them to adopt the fashion which is likely to please the 
examiners. 
In thermodynamics we cannot easily adopt Mr. Muir’s sug- 
gestion. Take the simplest case of unit quantity of mere fluid. 
v, p, t, and @ are such that they are all known if any two 
(except in certain cases) are known. Any one may be expressed 
as a function of any other two. My symbol (2) is quite 
adv |p 
definite. But to adopt Mr. Muir’s suggestion I must say :— 
= i 
Let E = fv, pthen fx, p is what my symbol means. Inas- 
much as my letters stand for the same quantities irrespective of 
the letters of which they are functions, I use one letter “ where 
on Mr. Muir’s suggestion I must use & as fv, p or Lv, ¢ or 
Wu, » or XP, ¢ or 6, » or &, g or six distinct symbols if 
T have to express any differential coefficient of #, and if I have 
to express all the differential coefficients of 7 I must use other 
six symbols ; altogether I must use thirty of these curious 
symbols instead of five common letters, and, furthermore, I 
must keep them all in my head. JOHN PERRY. 
The First Magnetician. 
WHILE thanking you and ‘‘R. T. G.” for the exceedingly 
kind appreciation of the Gilbert Club’s English translation of 
““De Magnete”’ (p. 249), I write to express the wish that the 
notice had mentioned the names of those who have collaborated 
in the production of this version. They are the late Mr. 
Latimer Clark, the late Sir B. W. Richardson, Rev. A. W. 
Howard, Prof. R. A. Sampson, Dr. Joseph Larmor, Sec. R.S., 
Prof. Meldola, F.R.S., Mr. Edward Little, Mr. G. T. Dickin 
and Rev. W. C. Howell. To the last-named a special recog- 
nition is due for indefatigable and critical care during the long 
final révision and ‘press correction. 
July 14. SILvAnus P. THomeson. 
“*Fox-shark’”’ or ‘Thrasher’? (Alopecias vulpes) in 
the English Channel. 
ON July 2a fine specimen of this shark was captured several 
miles south of the Eddystone Lighthouse by fishermen in 
search of mackerel. The fish was taken at a depth of about 40 
fathoms, and did a large amount of damage to the mackerel 
nets before it could be hauled on board and killed. The shark 
was brought to the Plymouth Museum and purchased for the 
collection. 
It may be worth while to state that the spiracles, which 
Couch says he was unable to detect, are distinctly visible in 
this specimen. It is scarcely surprising that they should be 
sometimes overlooked, for though our fish is 13 ft. (thirteen feet) 
7 in. (seven inches) long (of which the tail occupies seven feet), 
the spiracles are only 1/12th (one-twelfth) of an inch long by 
1/16th (one-sixteenth) of aninch wide. Each is situated exactly 
24 (two and a half) inches behind the eye, and a line from the 
spiracle to the tip of the snout passes just above the centre of 
the pupil. E. ERNEstT LOWE. 
Plymouth Museum, Plymouth. 
THE TRAMWAYS EXHIBITION AT THE 
AGRICULTURAL HALL. 
aye: International Tramways and Light Railways 
Exhibition which came to an end on Saturday last 
must be regarded as having been very successful from 
all points of view. The opening ceremony was per- 
formed by Mr. Gerald Balfour on July 1, and was 
accompanied by the usual luncheon and speeches. Mr. 
Gerald Balfour alluded, as might have been expected, 
to the recent deputation to his Department on the subject 
of electrical legislation, but he did not evince any sign 
of having become convinced of the necessity for speedy 
reform. In other respects the speeches were not of 
much interest ; the same may be said to be true toa 
certain extent of the proceedings of the International 
Tramways and Light Railways Congress, which held its 
meetings on July 1 and 2.. The Congress, which was the 
NO. 1707, VOL. 66] 
NATURE 
[JuLy 17, 1902 
twelfth held by the Union internationale permanente de 
Tramways, was the first to be held in London; the 
papers read and discussed dealt with the management 
and technical details of tramway schemes, and were 
most of them contributed by the engineers or managers 
of continental tramways. Many of them were very 
valuable, especially as they were based on the results of 
wide practical experience, but we doubt if they would 
prove of great interest to the readers of NATURE. 
The exhibition itself contained a number of very 
attractive exhibits. Although primarily a general exhibi- 
tion of all things pertaining to tramways, there was much 
on view which was of the greatest interest to those 
having nothing to do with traction. It was also very 
noticeable that the exhibition resolved itself practically 
into one of electric tramways. Of course, there was much 
that was not electrical—such, for example, as rails, 
points, &c.—but these are all part of the equipment of 
an electrical system. And perhaps the general impres- 
sion with which one left the hall, that a “tramway” was 
necessarily the same thing as an “electric tramway,” was 
of more interest, as a sign of the times, than were any 
of the individual exhibits. 
Several different types of car were on view; the one 
which, not unnaturally, attracted the most attention was 
that constructed by Messrs. Dick, Kerr and Co. for the 
London County Council. This is the first of one hundred 
cars being built for the Council’s South London Tram- 
ways. ‘The car is double-decked, and has a total seating 
capacity of sixty-six (twenty-eight inside and thirty-eight 
outside), and is equipped for the conduit system to be 
used on the South London lines. The Westinghouse 
Company exhibited a car which ran over a fully equipped 
trolley line laid along the total length of the hall, a distance 
of more than 300 feet. Power was obtained for running 
this from a 75 kw. direct-current generator (500 volts), 
driven by a Westinghouse three-cylinder gas engine. 
The car was fitted with the Westinghouse magnetic 
brake. This brake has atriple action, acting as a wheel- 
brake, a track-brake and an axle-brake ; it is energised 
by current derived from the car motors, which work as 
generators whilst the car slows down, the necessary 
energy being derived from the momentum of the car. 
The action of the brake is therefore independent of the 
main current supply. 
A notable feature of the exhibition was the Bremer 
arc lamp, exhibited by the Westinghouse Company. This 
lamp was used for part of the lighting at the Natural 
History Museum on the occasion of the Institution of 
Electrical Engineers’ conversazione. Unfortunately, it 
did not create a very favourable impression there, as the 
lamps kept flickering; those at the Agricultural Hall 
seemed to be burning much better. The carbons used 
in the Bremer lamp are saturated with certain minerals 
which volatilise and become incandescent in the arc; 
they are, moreover, arranged nearly parallel to one 
another instead of vertically one above the other ; the 
ends project a little below a protecting hood, meeting at 
an angle of about 20°, and the arc is kept at the tips by 
means of a magnetic deflecting device. The position of 
the arc, the materials used in the composition of the 
carbons, and the reflecting power of the conical hood, 
combine to produce a highly efficient light. It is said 
that the lamp is three times as efficient as an ordinary 
arc. The colour of the light is also much pleasanter 
and warmer than that of the ordinary arc, and the light 
appears to fill the globe much better, with the result that 
it produces somewhat the effect of a golden ball of 
light. 
Another similar arc lamp exhibited was that of the 
Union Electric Company. This, which is called the 
“Flame” arc lamp, has vertical carbons like an ordinary 
lamp; the carbons are, however, cored with a mixture of 
certain fluorides, and the upper one passes through a 
