—— 
_an effective braking action. 
NovEMBER 22, 1906| 
NATURE 
87 
not in good condition, but there appears to have been some 
doubt on this point; in fact, the balance of evidence is 
that the driver was mistaken. The driver also stated that 
he had had trouble with his hand brakes previously to 
the accident; the wheels, he said, seemed to skid directly 
the brakes were applied, and, when released, did not 
immediately revolve even when sand was used. 
It was a regulation of the company that all cars should 
be brought to rest at the top of the hill, but when the 
driver attempted to stop his car with the hand brake the 
wheels skidded, owing, as he said, to the rails being greasy 
from having been recently watered. Upon this he released 
the hand brake and tried the magnetic brake, but as the 
wheels continued to revolve the magnetic brake was use- 
less, and the result was that the car ran past the Archway 
without stopping, and came on to the gradient of 1 in 22. 
The speed having increased so that the car was getting 
beyond control, the driver signalled to the conductor to 
apply his hand brake, but this having no effect the con- 
ductor released it again. The driver then reversed his 
motor, thus causing the automatic switch to blow, after 
which he moved the controller handle to the position in 
which the motors would generate current against each 
other in order to produce a powerful braking action. 
These efforts, however, had little effect on the speed of 
the car, which dashed down the hill, with the terrible 
results before mentioned, until brought to rest by running 
into the empty car at the bottom of the hill. Before this 
the driver had jumped off, abandoning the car to its fate, 
his desertion being more disastrous as there was no one 
to ring the bell, a circumstance which, in Colonel Yorke’s 
opinion, led to the large number of persons being injured. 
The fact seems to suggest the need of an automatic con- 
tinuous striking bell which would be put in operation only 
upon emergencies. This would have the additional 
advantage of relieving the driver of one operation at times 
when he would be hard pressed. 
It will be gathered from what has been said that the 
electromagnetic brake is only brought into play when the 
motors are acting as generators, and therefore it evidently 
cannot be used when current is being supplied to the motors 
from the overhead conductor. The motors become gener- 
ators through the action of the road wheels, and, therefore, 
as soon as the latter cease to revolve the current required 
to energise the brake magnets ceases to be generated. 
This is the weak point of the arrangement, for if the 
hand brakes are put too hard on the wheels will skid, 
or be locked, and the rail brake become useless. The 
loss of the assistance of the magnetic brake owing to the 
skidding of the wheels is more serious because the fact of 
skidding reduces very greatly the retarding effect of brakes 
upon a car. 
The experiments made in 1878 on the Brighton Railway 
by Sir Douglas (then Captain) Galton, Mr. Stroudley, and 
Mr. Westinghouse are fairly well known to railway 
engineers. Apparatus was designed by Mr. Westinghouse 
by means of which, through water pressure and Richards 
indicators, there were recorded the retarding force which 
the friction of brake blocks exerted on wheels, the force 
with which the blocks pressed against wheels, and the force 
required to drag the van. These experiments clearly proved 
that when the wheels of a car are skidded, or blocked by 
the brakes, the retarding effect is very much less than 
when the brake shoes are pressed on the wheels with a 
force just short of that needed to cause skidding. The 
fact was known previously; indeed, in 1846 Mr. J. V. 
Gooch issued an order to the men on the South-Western 
Railway that wheels were not to be skidded; and the 
result might have been deduced from the experiments of 
Prof. Fleeming Jenkin on the effect of friction. 
Although a skidded wheel does not afford the same 
resistance to the forward movement of the car as does one 
which continues to revolve, yet the brake shoes must be 
pressed on to the wheel with sufficient force to produce 
¢ This action is by far the most 
effective just at the instant that skidding commences, 
there being then a very sudden rise in tangential resist- 
ance. Just at the moment the brakes are released— 
the wheels being skidded—there is another rise in tan- 
gential force caused by the brake blocks. Prof. 
Fleeming Jenkin’s experiments on the effect of friction at 
NO. 1934, VOL. 75] 
different speeds may be consulted with advantage in con- 
nection with these results. Although the ordinary brakes- 
man does not carry out quantitative experiments by the 
aid of elaborate apparatus, he finds by experience that his 
brakes are most effective when the critical point is 
approached. The most skilled men will manipulate their 
brakes with great effect, getting the greatest retarding 
action for the car without skidding the wheels. The best 
way in which to work brakes, therefore, is to apply a 
considerable force at first, releasing it as the skidding point 
is almost reached. 
Another point in connection with brake action, which 
almost follows from what has been said, is that although 
a good deal of pressure on the blocks is needed to make 
a wheel skid, a comparatively moderate force is needed to 
keep the blocks on when the wheels have once stopped 
revolving. Still another point bearing on the question 
under consideration is the decrease of friction that takes 
place with increase of speed of movement between rail and 
wheel. This is contrary to what is observed with lubri- 
cated surfaces, but, as Sir Alexander Kennedy has pointed 
out, it bears out the smaller experiments of Prof. Fleeming 
Jenkin. On the Brighton Railway Company’s trials the 
effect was clearly proved. The experimental van was drawn 
alone by a powerful express engine, and was thus able to 
maintain a high speed with the brake on, and it was clearly 
shown that there was greater adhesion between rails and 
skidded wheels at high speeds than at low speeds. In some 
cases the tangential-force diagrams showed a rise in 
adhesion of too per cent. 
The bearing of these facts on the present case is plain. 
That a driver of a car will go as near skidding as possible 
is apparent, and an unskilled man will often pass the 
critical point. Then the wheels will cease to revolve, and 
no current will be generated to energise the electro- 
magnet; consequently the rail brake will be out of action, 
and, as a skidded wheel does very little to check the 
momentum of the car, all the elements of a serious catas- 
trophe are present when descending any considerable in- 
cline. Beyond this, the rail brake cannot hold a car 
stationary on a hill when once it has been brought to 
rest. 
These defects would be overcome if the main current 
from the overhead conductor were available for energising 
the electromagnet. This would introduce some compli- 
cation and extra fittings, but there does not appear to be 
any insuperable difficulty. The fact that the present 
electric-rail brake is liable to fail just when it is most 
needed—as shown by the Highgate tragedy—and the re- 
membrance of the terrible results of a heavy car rushing 
down uncontrolled amongst traffic, make it plain that con- 
siderable sacrifice is warranted if the powerful rail brake 
can be brought more readily into play at a time when it 
is most efficient, namely, before the travel of the car has 
acquired a high velocity. 
The particulars we have already of the accident form a 
practical illustration of the bearing of the experimental 
data collected on the Brighton Railway trials. Whether 
the skidding of the wheels of the car—which undoubtedly 
took place, as flats were afterwards found on the tread of 
the wheels—was due to want of. skill on the part of the 
driver or to injudicious rigging of the brake is a matter of 
interest rather than importance, for drivers are as liable to 
be flurried or unskilful as brakes are likely to be improperly 
rigged. In regard to the first proposition, the driver’s 
training in the present case consisted of eleven lessons of 
about one hour each in a school, and twelve lessons on 
the road. This appears to have been considered sufficient 
instruction to entitle the driver to hold a certificate of 
“thoroughly instructed in the duties of a motor-man and 
now competent to take charge of a car.’’ After that he had 
three days’ practice on the Archway route with another 
motor-man, including one day’s instruction with the motor 
brake. He had been in regular work for twenty days at 
the time of the accident. 
We are not aware whether this driver had had any 
mechanical training, or had been employed about the 
mechanism of motor vehicles, before he began his driving 
career, but if not the course of instruction appears in- 
sufficient. This was borne out by his evidence at the 
inquest, for he was not aware that the magnetic brake 
