Part 42
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 ship 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 ship 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 ship 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 ship 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 ship and cradles, a weight of 12,000 tons.
+------------------------------+-----------------------------+
Force in tons required
to move or restrain ship 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 ship on
similar ways, but on the level
+------------------------------+-----+------+-----+-----+-----+-------+
Velocity, feet per second
0
0 to 1
75
1
15
2 to 3
+------------------------------+-----+------+-----+-----+-----+-------+
RAILS AND CRADLE BARS.
{ 890
No. 1. Ample lubrication
..
..
1,060
1,000
{ to
{ 800
No. 2. Medium lubrication
1,220
..
..
1,000
940
No. 3. Very little lubrication
1,400
..
1,200
..
1,000
..
{
1,560
No. 4. No lubrication {
to
..
..
1,000
..
{
1,400
+------------------------------+-----+------+-----+-----+-----+-------+
In every case where a velocity approaching to 3 feet per second was attained, whether the ways were lubricated or were quite dry, the model, though there was no tractive force acting on it other than that of gravity down the incline of 1 in 12, rapidly increased its speed till it reached the end of the ways.
These experiments are worthy of note for the contradiction, already referred to, which they gave to the received rules relative to friction. It will be seen by these experiments, and as will hereinafter appear from the results of the movements of the ship herself during the launch, that with different degrees of velocity very great variation in the friction was apparent, amounting to a difference of about thirty per cent. in the case of unlubricated surfaces, according as the velocity was nearly _nil_ or was 15 feet per second, which is a comparatively small velocity. The friction in this case was, on a weight of 12,000 tons, 1,500 tons at a very low velocity, and but 1,000 tons at a velocity of 15 feet a second or about one mile an hour, the friction at the very low velocity being fifty per cent. greater than that at one mile an hour.
As soon as the ship began to move by slides, the recording apparatus of tape and pendulum was applied to record the nature of the ship's movements. This apparatus was similar to that already described. The tape was attached to the bottom of the ship under her centre of gravity, and recorded the rate of r.e.t.a.r.dation of the ship when left to herself after the motive appliances had, with the exception of gravity, ceased to act; and the amount of friction acting to r.e.t.a.r.d the ship was determined with a very considerable amount of accuracy.
The best experiments made were on December 7, 8, and 10; and the results are very interesting. The dirt and rust of the sliding surfaces had increased the adhesion very much, and a considerable force was necessary to start the ship. There being no good pressure gauges to the presses, it was impossible to decide exactly what was the force required to start the ship, for of course the tape record gave no information on this point; but there is no doubt it was considerable, probably 800 or 1,000 tons in addition to gravity, and was thus far greater than the force to start that had been observed with the experimental cradle.
But the remarkable fact was that, notwithstanding the deterioration of the sliding-surfaces as evinced in the increased difficulty in starting, the friction, when once motion was established, was proved not to be very largely in excess of that which had been exhibited in the experiments.
In the various experiments tried, it was shown that when the ship had a velocity of between 6 and 8 inches per second, the amount of friction was only about 100 tons in excess of the action of gravity down the incline; while as the velocity became less, the friction became greater, till, as the velocity became smaller, the friction increased from 200 to 300 tons in excess of the action of gravity.
The results obtained by the observations made on the motion of the ship having shown that its behaviour when in motion accorded with that of the experimental cradle, there is every reason to believe that if the ship had ever attained a velocity of 1 to 2 feet per second, which might have happened had the river tackle acted well, the friction would, as in the experimental cradle, have become less than the action of gravity down the slope, and the brakes would have had to be employed to check the motion.
NOTE B (p. 384).
_Letter from Mr. Brunel to W. Froude, Esq._
February 2, 1858.
My dear Froude,--It is no news to you to tell you that we have floated, but still you will perhaps feel sympathetically some pleasure in hearing of it from me, as I do in writing to you upon it.
We have in fact gone on well and without mishap since we have resumed operations with plenty of power; we have not gone very quickly because our jumps have been small, or we have gone by a continuous motion--we have had a great deal of this, and all the last 30 or 40 feet I think, or more, has been so, the power being _with_ gravity about a quarter of the weight, sometimes less--occasionally, when the water was high, considerably less--buoyancy being of course taken duly into the calculation.
Once, when still weighing fully 3,500 tons, and with 1,200 tons of water in her, making 4,700, less some buoyancy of the cradle, she moved so easily that they came running to me from the other cradle, to say that she was moving of herself, and asking what to do.
She certainly had not much more pressure on than we had a.s.sumed to be necessary to overcome all the friction of thrust timbers, &c., certainly could not have had above 300 tons of real push to move
2,000 2,000 tons, 2,000 -- ----- (gravity) = 1,833 tons.
12
I think that when the load became much lighter on the rails, that the mud and even the sand of the Thames form a lubricator and _rollers_ which offered less resistance than the dry rail, or the rail with pressure enough to displace the mud or imbed the sand.
Having at last pushed the cradles beyond the rail, and found her stand well, and moved a few feet more and still stand upright, I waited for a tide, and arranged a good communication with Liverpool and Plymouth to telegraph up wind and weather, morning and night, so as to help in foreseeing a tide.
On Friday morning, at 3 A.M., the tide began to improve, but the wind was still in the wrong quarter for a good tide, and as I think I have told you the tides of this month, and of the whole year of '58, are very poor. Besides this, it blew a gale. I therefore began pumping out the water, but stopped at 1,200 tons, and at 10 A.M., seeing no improvement in the weather, I filled in by the fire-engine about 1,300 or 1,400 more, and gave up the attempt, and very fortunately, as the tide did not rise high enough, and the wind right on her broadside increased.
On Sat.u.r.day night things grew worse, and the wind at Liverpool and Falmouth finished in the evening still SSW. About midnight the wind lulled, the rain came down in torrents; the wind gradually stole round to the northward, and the tide came rolling up uncomfortably quick. I admitted water as soon as it would run in, and only just prevented her floating; when, if she had, we should have been in a mess, as our wedge-pieces were not yet unbolted from the bottom, nor could we in the night have managed the floating, even if all had been ready, which nothing was. Under these circ.u.mstances, the wedge-piece being bolted was perhaps our security, as she all but floated; the stern rose 3 inches though we had 3,000 tons of water in her, the tide rose so high; we began pumping immediately on the turn of tide, and by half-past 6 A.M. the wind was gentle from the NNE., and telegraphs came to announce the same in Liverpool, &c. I therefore determined upon floating.
We pumped away, and at 10 A.M. cleared everything away, and began our preparations. I had fixed 12 o'clock to begin forcing her further down, thinking half-past 12 would have been as early as she would have moved easily, but the tide came rushing up an hour and a half before its time; and, although I hurried the men to their posts, we were rather caught napping. The instant we began pressing, she moved easily, and by a quarter past 1 we had pushed her centre past the ways, and she began lifting with the tide.
I had a centre station with a chalk board, giving me the curve of the tide, and two good levels reading off her stem and stern water marks. Our calculations had proved very correct. In the skew position I had pushed her to, she began lifting at the stern first, as she ought to have done, and her bows soon followed the example.
We stopped pushing the cradles, made them fast, and began hauling out the ship--she moved very slowly of course.
When she began to move out, our first difficulty occurred by a little mismanagement, or over confidence, on board; our stern chain was let go, and she forged ahead against the tide by the elastic strain of the bow chain, and _I think_ also by the tug-boats pulling too much, which ought not to have been done, and in going ahead her port paddles caught hold of some cradle timber rising, with long 2 inch bolts holding them to the bottom of cradle. No tug power would have had any effect upon them. But we had two hours before us, with a promise of an unusual tide; I hurried on board, and we succeeded in about twenty minutes in getting a little astern a little out, and getting clear.
We had some mishaps after this, such as fouling one of our barges, getting it jammed under the paddlewheels, and the barge fast by the chain tackle dropt overboard. However, we scuttled and sunk the barge, and got safely across. We had an extraordinary tide, and several a.s.sistances from nature to counteract any of our own bungling, and got safely across, and she is now moored in her right place.
Yours faithfully,
I. K. BRUNEL.
William Froude, Esq.
CHAPTER XIII.
_STEAM NAVIGATION--THE 'GREAT EASTERN' STEAM-SHIP._
_COMPLETION AND SUBSEQUENT HISTORY._
A.D. 1858-1859: aeTATIS 52-54.
A.D. 1859-1870.
PREPARATIONS FOR COMPLETING THE SHIP--FORMATION OF THE GREAT SHIP COMPANY--MR. BRUNEL'S ABSENCE FROM ENGLAND--PROGRESS OF THE WORKS FROM HIS RETURN TO HIS LAST ILLNESS--VOYAGE TO WEYMOUTH--EXPLOSION OF WATER-HEATER--STORM AT HOLYHEAD--DESCRIPTION OF THE SHIP--HER FIRST VOYAGE TO NEW YORK, JUNE 1860--SECOND VOYAGE TO NEW YORK, MAY 1861--VOYAGE WITH TROOPS TO QUEBEC, JUNE 1861--FRACTURE OF RUDDER-HEAD AND DESTRUCTION OF PADDLEWHEELS, SEPTEMBER 1861--VOYAGES IN 1862--ACCIDENT OFF MONTAUK POINT, AUGUST 27, 1862--VOYAGES IN 1863--FORMATION OF THE GREAT EASTERN STEAM-SHIP COMPANY--REMARKS ON THE HISTORY OF THE 'GREAT EASTERN' PREVIOUS TO HER EMPLOYMENT IN LAYING SUBMARINE TELEGRAPH CABLES--TELEGRAPH EXPEDITIONS OF 1865 AND 1866--FRENCH CABLE EXPEDITION OF 1869--VOYAGE TO BOMBAY AND ADEN, 1869-70--CONCLUDING REMARKS--_NOTE_: DIMENSIONS OF THE SHIP AND ENGINES.
