Part 42 (1/2)
NOTE A (pp. 348, 368, 372).
_Experiments and Observations on Friction._
In January 1857, Mr. Brunel took steps to form an estimate of the amount of hauling or of r.e.t.a.r.ding force that would probably be required in the launch.
Two rails were laid at an inclination of 1 in 12, and upon them an experimental cradle was placed, with three cross-bars similar to those which were to form the under surface of the cradles. The three cross-bars therefore made six intersections on the two rails, and the small cradle was loaded with about 8 tons, so that the weight on each intersection was about equal to that which would come on each of the intersections of the actual cradle. This arrangement was not therefore a model, but a correct representation of a part of the cradles, and which might, with an exception to be noted presently, be taken to exactly represent, by its conduct, the conduct of every similar part of the actual cradles. Experiments were made with one or two kinds of unguents, and, what was a more correct representation of what was likely to occur, with the rails and bars clean but not bright, and without lubrication.
The experiments with lubrication were useful rather as comparing the various lubricants one with another than as representing, by a mere process of multiplication, what would be the behaviour of the s.h.i.+p on her cradles, because, for the reason already pointed out in the case of wooden sliding-surfaces, the lubrication would be more and more rubbed away as more of the cradle pa.s.sed over it; thus the experimental cradle, when tried with lubrication, represented rather the behaviour of the front part of the cradle than that of the whole. Had the s.h.i.+p herself been moved uninterruptedly down the ways, the state of things would have been something between good lubrication and none at all. As the under sides of the bars were lubricated, any motion of one end of the s.h.i.+p before the other would tend to move the bars sideways over the rails, and so to spread the lubrication, and to pick it up and re-deposit it. Mr.
Brunel thought but little of the black-leading of the ways, considering that it would be rubbed off by the leading bars of the cradles; but a very little lubrication on metal surfaces is sufficient; and doubtless, had the s.h.i.+p been moved continuously down the ways, considerable a.s.sistance would have been derived from the lubrication which was applied.
The results of the experiment were curious. The generally received notion is, that friction between rubbing surfaces is independent of the velocity; that is to say, that whether a body be moving fast or slow within reasonable limits, the r.e.t.a.r.dation due to friction is the same; that if a body be sliding at a given velocity, whether that velocity be great or small, a drag of a certain number of pounds will keep it moving at that velocity. It was, however, always understood that a greater force was necessary to start a body from rest, to overcome adhesion. The experiments made with the experimental cradle distinctly showed that any rule as to friction being constant at different velocities was untrue. It was evident that, as the speed increased, the power required to overcome the friction became less. No exact records are extant of the experiments made with this experimental cradle before the launch; they were, however, repeated during the launch with great care, and the results very carefully a.n.a.lysed. The experiments showed generally that the tractive force, including the action of gravity, was never more than ?, or less than 1/15, of the weight.
Although the experiments showed that the amount of friction in the case of the actual launch would lie between the limits above mentioned, they at the same time indicated that it would not probably approach either of those limits.
Shortly after the commencement of the launch, Mr. Brunel had the experimental cradle and ways re-erected. A very simple arrangement was fitted up, by which the forces at work at each period of the progress of the cradle in each experiment might be deduced. The results of these experiments, which, as may be supposed, were similar to those obtained in the commencement of the year, were most instructive; they showed quant.i.tatively the decided diminution in friction which took place as the velocity increased, and the amount of that diminution. The apparatus was very simple. The experimental cradle, which has already been described, was made to slide down its ways by a chain attached to a suspended weight. The weight employed was generally about 5 cwt. After the cradle had run a certain distance, the weight reached the ground and the cradle proceeded with the momentum it had obtained. The velocity given to the cradle down the ways was measured in the following manner. A long piece of tape was coiled round a reel placed at the top of the inclined rails or experimental ways, so that it could revolve freely and pay out the tape as required. One end of this tape was attached to the cradle, so as to be drawn after it as it ran down the rails. The tape, as it ran off the reel, pa.s.sed over a guiding board over which swung transversely a pendulum arranged to swing once every quarter of a second. At the lower end of this pendulum was attached a brush which was filled with paint; and as soon as the model cradle moved, the pendulum was set oscillating by a self-acting trigger arrangement. The pendulum in its oscillations made marks on the tape as it ran out at every quarter of a second of time. Thus, by an examination of the tape, could be determined the exact distance which had been pa.s.sed over by the cradle during each quarter of a second of the time during which it was moving.
The rate of progress being thus known, and the actuating force (gravity acting on the cradle and on the suspended weight) being also known, it will be understood that the exact amount of the resisting force, namely, friction, could be calculated exactly, and this for each moment and position of the descent of the experimental cradle.
The following results of these experiments were recorded in terms of the corresponding amounts of tractive force that would be required to produce similar results in the case of the s.h.i.+p and cradles, a weight of 12,000 tons.
+------------------------------+-----------------------------+
Force in tons required
to move or restrain s.h.i.+p on
incline of 1 in 12
+------------------------------+---+------+---+--+---+-------+
Velocity, feet per second
0
0 to 1
75
1
15
2 to 3
+------------------------------+---+------+---+--+---+-------+
RAILS AND CRADLE BARS.
r.e.t.a.r.d-
ing
{ 110
No. 1. Ample lubrication
..
..
60
0
{ to
{ 200
No. 2. Medium lubrication
120
..
..
0
60
No. 3. Very little lubrication
400
200
..
0
..
{
560
No. 4. No lubrication {
to
..
..
0
..
{
400
+------------------------------+---+------+---+--+---+-------+
+------------------------------+--------------------------------------+
Force in tons required to move
a weight equal to the s.h.i.+p on
similar ways, but on the level
+------------------------------+-----+------+-----+-----+-----+-------+
Velocity, feet per second
0
0 to 1
75
1
15
2 to 3