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CONDUCTED BY JAMES QUICK. 
GLasGow UNIversity.—Prof. Andrew Gray, Lord 
Kelvin’s successor, delivered his introductory address 
on October 20th. It was a great honour and privilege, 
he said, to return to his old university and again to 
teach in the department of natural philosophy. He 
then went on to speak of the inter-connection of theory 
and practical applications in physics, referring to the 
ermanent work of such men as Galileo, Bacon, 
ewton, Maxwell and Hertz, the results of which 
work are so beneficial at the present time. In con- 
clusion he ssid that Glasgow University was very 
advantageously placed with respect to physical science, 
and in that University they wanted to give a new 
example, on a great scale, of the inter-cornection of 
theory and practice on which he had particularly 
addressed them. 
WIRELESS TELEGRAPHY IN. SoUTH AFRICA.—As 
a result of an arrangement made between the Goyern- 
ment and the Wireless Telegraph and Signal Com- 
pany, six sets of apparatus were dispatched from 
Liverpool on November 2nd for Capetown. These 
are to establish telegraph stations for transmitting 
war news in South Africa. Theyare under the charge 
of five assistants from the staff of the company, the 
Government also, having sent an engineer. 
PuysicaL Society or Lonnon.—The first meet- 
ing of this Society for the present session was held on 
October 27th, Prof. W. E. Ayrton, F.R.S., Vice- 
A President, in the chair. Dr. S. W. Richardson read 
, a paper on ‘The Magnetic Properties of the Alloys 
of Iron and Aluminium.” The author experimented 
upon four alloys, the percentage of aluminium ranging 
from 3°64 to 18°47 and the temperatures of the experi- 
ments from —83° C. togoo®C. Mr. G. L. Adden- 
brooke exhibited a model illustrating a number of the 
actions of the flow of an electric current, and Mr. W. 
Watson showed some of Prof. Lecher’s experiments 
with the Wehnelt electrolytic break. It was very 
clearly seen by these experiments that subsequent 
sparks tend to pass through the portion of air heated 
by the first one. 
A PROBLEM IN PHysics.—Some years ago, one 
of your contemporaries offered a prize for the best 
answer to the question, Does a ball thrown straight 
up in the air occupy the same time in descending as 
in ascending? I thought the matter out, and sent in 
my reply arguing that it would not fall so quickly as 
it rose; but the prize was given to someone who 
argued that the times would be equal. On discussing 
the matter afterwards with a well-known physicist, I 
found that he too was under the impression that the 
times would be equal ; and in view of this apparently 
general impression, it occurs to me that your readers 
may be interested in the following, which I think is a 
complete demonstration that the times would not be 
equal, the descent occupying longer time. As _ev 
student of Physics is aware, a ball thrown vertically 
upwards in a perfect vaccum, with sufficient initial 
velocity to carry it to a height of 50 feet, would 
occupy the same time in falling as in rising. — Its 
gradually increasing velocity when falling would be 
SCIENCE-GOSSIP. 
217 
the exact counterpart of its gradually decreasing 
velocity when rising, so that at the end of the fall its 
velocity would be the same as at the beginning of the 
rise. When, having te overcome the resistance 
of the atmosphere, however, the ball, though 
thrown with the same initial velocity, would 
not rise so high, say only go feet. We have already 
seen that it requires a fall of 50 feet in a vacuum to 
reproduce its initial velocity, from which it is evident 
that the fall of go feet, opposed by the resistance 
of the atmosphere, will be insufficient to give it this 
velocity. Now, consider the ball at 10 feet from its 
starting point. It has a velocity that would take it to 
a certain height in a vacuum, but the resistance of the 
atmosphere prevents it rising so high. Arguing as 
above, it can be shown that when the ball returns to 
this point, its velocity will be less than at the same 
point during its rise, and similarly for every other 
point. That is to say, at no point during the fall is 
the velocity so great as at the same point during the 
rise, which of course means that the fall will take 
longer.—W. F. Dunton, 60, Cathles Road, Balham. 
[The point is interesting and Mr. Dunton’s argu- 
ment is probably sound. In vacuo, of course, as Mr. 
Dunton states, there being no air resistance, the times 
of ascent and descent are the same. In air, however, 
the air resistance always acts as a negative force and 
therefore produces an acceleration opposing both the up 
and down motion. Suppose a be the acceleration due 
to the air friction, ¢ that due to gravity, then when the 
body is ascending the acceleration reducing the motion 
will be —(g+-2) ; on the downward journey it will be 
g—a. Now the space s travelled each way is the same, 
therefore from the equation s=} /?? or /= nl 25 
(where /is the total acceleration) we have in the first 
25_and in the second case ¢,= \/ —75_ 
g+a g—a 
case 4; = Mf 
z, is therefore less than ¢,. The above assumes that 
the air friction is constant for the varying velocities of 
the body. This assumption is not strictly true, the 
friction increases as the velocity decreases, but not in 
any simple ratio. Further communications on this or 
any other physical topic, will be welcomed for dis- 
cussion in these columns. —J.(Q. | 
Monocnromatic Licur.—In a recent number of 
‘Comptes Rendus,” a description is given of 
MM. Fabry and Perot’s work upon monochromatic 
lights. Although the sodium flame is an exceedingly 
useful source for general purposes, and is so easily 
obtained by a little common salt inserted into any 
convenient flame, yet for some particular purposes it 
is at a disadvantage. Especially is this felt at times 
in photographic work, as the sodium lines appear in 
a part of the spectrum to which the ordinary photo- 
graphic plate is practically insensitive. MM. Fabry 
and Perot’s arrangement, although more complicated 
than that for the ordinary sodium flame, permits of 
obtaining violet, yellow and green monochromatic 
rays. An electric arc is produced, in a Torricellian 
vacuum, between two mercury surfaces in the two 
limbs of an inverted U tube. The arc is obtained by 
shaking the tube, thus completing the circuit 
momentarily by a splash of the mercury. The violet 
rays produced may be cut off by interposing yellow 
glass, or a thin layer of bichromate of potassium. 
The green and violet rays may be eliminated by a 
solution of eosine, and the yellow rays by a strong 
solution of didymium chloride. To obtain the green 
rays only, a mixture of didymium chloride and 
potassium bichromate, may be used. 
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