104 
rorULAIl SCIENCE REVIEW. 
(5.) When two simple tones are led singly to the ears no differential tone 
is heard. There is some evidence that summational tones are heard. 
(6.) In binaural audition dissonances are excessively disagreeable, and 
consonances harsh. 
The Stability of Cleopatra's Needle. — A curious controversy has arisen in 
the columns of the “ Times,” and other papers, on this subject. The idea of 
any real instability no doubt arose from the unusual form of the monument,, 
and the common error in estimating its dimensions. It seems the section is 
nowhere less in diameter than five feet ; and, that being the case, it has been 
computed that it is competent to stand a wind pressure of 80 to 90 lbs. per 
square foot. It is said, however, that on January 30, 1868, at the Liverpool 
Observatory, the anemometer, which registers up to 60 lbs., was driven far 
beyond its limit, and that the pressure registered could not have been less 
than from 70 to 80 lbs. on the square foot. It is stated by the authorities 
that the maximum wind pressure in this country does not exceed 55 lbs. on 
the square foot. Mr. Dixon illustrated the subject by comparing the alleged 
pressure to the weight on the floor of a crowded room, which can attain from 
80 lbs. to 120 lbs. ; the latter with labourers of large stature packed as closely 
as possible. Thus, he remarked, as the windows certainly have to bear an 
equal strain with the walls, would anyone dream of standing on a floor formed 
of glazed window sashes ? This reductio ad absurdum was, at first sight, 
conclusive against the accuracy of the anemometer. But Professor Unwin 
points out that, taking a plate of glass 2 feet in diameter, |-inch thick, and 
loaded with 80 lbs. per square foot, we get the greatest stress as = 4270 lbs. 
per square inch. Now the tenacity of glass has been found to be from 4,200 
to 6,000 lbs. per square inch. Hence it is really possible that a pane of 
2 feet in diameter might sustain the supposed load, if uniformly distributed, 
without breaking. 
Mr. Dixon further declares that 130 lbs. of wind pressure would not 
upset it. This estimate, however, seems to need modification. “While,” 
says a contemporary, u if a pressure of 80 lbs. is reached, it is very ques- 
tionable if the survivors among the inhabitants of the neighbourhood will 
find it in situ when they have time to go to look for it.” 
On the Illumination of Lines of Molecular pressure , and the Trajectory of 
Molecules. — Mr. William Crookes read an important paper before the Royal 
Society on December 5 upon the above subject. 
He has examined the dark space which appears round the negative pole 
of an ordinary vacuum tube, when the spark from an induction-coil is passed 
through it. The following are some of the propositions he arrives at : — 
1 . Setting up intense molecular vibration in a disc of metal by electrical 
means excites molecular disturbance which affects the disc and the sur- 
rounding gas. With a dense gas the disturbance extends only a short dis- 
tance from the metal ; but as rarefaction continues, the layer of molecular 
disturbance increases in thickness. In air at a pressure of *078 millimetre 
this disturbance extends for at least 8 millimetres from the surface of the 
disc, forming an oblate spheroid around it. 
2. The diameter of this dark space varies with the exhaustion ; with the 
kind of gas in which it is produced ; with the temperature of the negative 
pole ; and, in a slight degree, with the intensity of the spark. For equal 
degrees of exhaustion it is greatest in hydrogen, and least in carbonic acid. 
