TRANSACTIONS OF THE SECTIONS. 111 
the point passes several times over different parts of the same line of Jetters, an exact 
copy of the written communication is made, the letters appearing of a light colour 
on a background of closely-drawn spiral lines. There are numerous electrolytes 
adapted to mark the paper, but the one that has hitherto been found most available 
is the prussiate of potash dissclved in a diluted acid. Very complicated chemical 
actions ensue when the electric current passes from a steel point through paper sa- 
turated with such a solution, the result of which is to leave a stain of prussian blue 
on the paper. When the paper is moistened with diluted acid alone, the message is 
impressed invisibly on the paper, and it is brought out by afterwards passing the 
paper through a solution of prussiate of potass. By the arrangements described 
copies of writing may be made at any distance to which an electric current can be - 
conveyed, provided the two instruments are moving exactly together. One of the 
alleged advantages of this telegraph, as compared with needle telegraphs, is that it 
will be free from the perturbing influence of atmospheric electricity. By another 
invention connected with the copying telegraph, independent connections with dif- 
ferent stations and with branch lines may be obtained. The cost of the instrument 
would be 301. 

On the Britannia Bridge. 
The President (Mr. R. Stephenson, M.P.) gave an account of the causes which 
produced the late accident, and of the difficulties which have stood in the way of 
finally completing the work. 
1 
On a Machine for Ventilating Coal Mines. By Witi1AM Brunton, C.B. 
Although this paper referred to rarefaction as a means of ventilation, and not to 
the manner-in which the air is coursed or conducted through a colliery, the author 
briefly noticed the principle upon which the air is conducted in the best ventilated 
collieries, that is, by dividing the workings of the colliery into districts, so that in no 
case does the air traverse through the whole of the workings, but being apportioned by 
the wasteman to each of the districts through regulating doors, and passing through 
the internal ramifications, the air of each district is ultimately and separately dis- 
charged into the waste or return air-course which conveys it to the upcast shaft. 
_ As the tendency of the current is to take the shortest course, it becomes necessary 
to retard the current of the shortest, or first district, in order to ventilate the second, 
and the second to ventilate the third; and, in short, it is by judicious retardation 
that the air is made to pass efficiently through all, and more especially through the 
last or longest district. 
Though the division of a colliery into districts is found, upon the whole, a very 
great improvement upon the original method of coursing all the air through the 
whole extent of the works, yet it is manifest that it has rather increased the neces- 
sity for care, skill, and constant supervision on the part of the wasteman, and under 
particular circumstances presents less security against explosion. 
’ The author described the furnace (the ordinary power of rarefaction) for which 
the machine he has invented is intended to be a substitute, and proceeded to point 
out wherein that mode of rarefaction is defective and ill-adapted to the purpose. 
_ Having thus endeavoured to state the nature of the furnace, and also what he 
conceives to be its inherent defects, he entered upon the description of the apparatus 
which he has invented and applied as a substitute. He drew attention to the prin- 
ciple of its action, namely, centrifugal force, and also to its construction—an inte- 
gral drum, with radial or curvilinear compartments supported and revolving upon a 
vertical axis, whereby the air contained in the compartments is discharged with a 
force (and a corresponding measure of rarefaction is produced in the central part of 
_ the drum) due to the velocity with which it revolves. 
__ Its construction is of the most simple character; it has no valves or separate 
moving parts, has no attrition, and all the friction is resolved into a foot pivot 
_ Moving in oil: when at rest, it presents no impediment to the air ascending the pit, 
__is very inexpensive, and liable to no derangement. In short, it is a simple in- 
tegra] implement, whereby any degree of rarefaction necessary to the ventilation of 
a colliery is rendered certain and regular, under all the changes that so injuriously 

