SCIENTIFIC SUMMARY. 
97 
those in the other direction, and the current of air is a steady continuous 
current always flowing in one direction. Along the line of pipes are stations 
at which carriers containing messages can he introduced into or taken out 
of the circuit, and a beautiful piece of apparatus has been contrived by which 
the carriers can be introduced into or taken out of the tubes almost instan- 
taneously, and without in any way interfering with the steady flow of the 
air-current. In order to prevent any possibility of ike blocking up of the 
tubes by two carriers jamming, an electrical system is adopted, like the block 
system of railways, by which each station telegraphs on the arrival and 
departure of each carrier, and the station to which it is sent prepares to 
receive it. Although, of course, the passage of the carriers through these 
tubes is very slow compared with the speed of electricity, yet as some thirty 
messages fully written out can be sent in each carrier, it is found quicker 
and cheaper to use pneumatic tubes than to use electric wires in cases where 
the number of messages to be sent daily is very great. Mr. Siemens even 
thinks that in large towns we shall ultimately employ the pneumatic system 
to distribute the letters from the Branch to the General Post Office, and 
vice versa. He thinks that if a complete pneumatic system were in operation 
in London between the post offices, every letter might be delivered at its 
destination an hour after being posted. 
As a subsidiary invention Mr. Siemens has perfected a new form of steam- 
jet for exhausting the air from the tubes. As an engine with its boilers 
and air-pumps is an expensive item in the erection of a system of pneumatic 
tubes, it appeared to him that some cheap substitute was desirable, even if 
its efficiency were less than that of an engine and air-pumps. He has found 
such a substitute in the well-known steam-jet, but he has so improved that 
instrument that now its efficiency appears to be very little less than that of 
an engine with exhausting cylinders. 
MEDICAL SCIENCE. 
Abnormal Larynx . — Sir Duncan Gibb described some very curious forms 
of the above at the late meeting of the British Association. In one he 
described a rare instance of absence of both arytenoid cartilages in a girl of 
18. Likewise, one in which the epiglottis possessed the shape of a trefoil 
leaf, and two others in boys of fissure of the same cartilage. 
The Ohio Twins . — These are said to surpass in peculiarity the Siamese twins 
and the “ Two-headed Nightingale,” recently exhibited in town. The 
“ Lancet ” says, that the Ohio twins have been examined by Drs. Williams 
and Little, and they are described as being united in a direct line from 
the occiput downwards along the spine. On one side are perfectly developed 
hips, thighs, legs, and feet. On the other side there is one large, imperfectly 
formed leg, presenting the appearance of the consolidation of two legs... 
There are eight toes on this limb, two of which have the appearance of great 
toes, being much larger than the others. Each child has a well-formed 
head and features, good arms and hands, lungs, heart, liver, stomach, &c. 
The lower portion of the trunk is said to be common to both. While these 
YOL. XI. — NO. XLII. H 
