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is going ahead, but her engines reversed.

"The experiments made by Professor Reynolds, of Manchester, in reference
to the correct steering of screw steamships, when going ahead with the pro-
peller working astern, and the results of the trials made with the steamer
' Melrose,' which have been published in the Glasgow News, have induced us,
the undersigned, to try the three chief manosuvres in question with the
steamers which we command. We subjoin a statement of the results obtained,
accompanied by sketch and explanation.

"As screw steamers differ from each other in respect of model, construc-
tion, and size of propeller and helm, draught of water, &c., there is naturally
a difference in the degree in which they deviate from a straight course when
making these movements. We would, therefore, recommend every master to
make experiments with his ship, with a view to ascertaining in what way the
helm should be handled in all conceivable emergencies.

"I. Ship going ahead, propeller working astern, rudder amidships.
"Result. The stern turns to the right.

"Explanation. The rotation of the screw to the left presses the stern to
the left, and consequently the stem to the right; the helm, which is amid-
ships, is thus neutralised, and must be regarded merely as a prolongation of
the ship.

"II. Ship going ahead, the screw working backwards and the helm hard

"Result. The stern turns very decidedly to the right.




"Explanation. The rotation of the screw to the left presses the stern to
the left, and consequently the stem to the right ; the starboarded helm is
subjected to a pressure of water from behind, so that the after part of the ship
is doubly impelled to the left, the fore part being also with correspondingly
greater force pressed to the right.

"III. The ship going ahead, propeller working astern, helm hard a-port.
"Result. The stern inclines slightly to the left.

"Explanation. The rotation of the screw to the left presses the after part
of the ship to the left ; on the other hand, the ported helm is under a pressure
of water from behind which impels the stern to the right. The operation
of these two forces being of opposite nature, the vessel only deviates
slightly from the straight course, and then mostly with the stern to the

"But as soon as headway is lost the force of the screw asserts itself
so much more that, in spite of the helm being ported, the stern turns to the
left and the bow to the right ; they turn in the same directions in a stronger
degree when the helm is amidships, and strongest of all when the helm is
starboarded. From the moment the ship begins to go astern the screw must
be regarded as the fore end or bow of the ship. After the propeller, on the


order ' Full speed astern ' being given, has made some few revolutions, there
comes a short period during which the helm can be moved to either side with
almost the same facility as when the vessel is lying stationary, after the


expiration of which it almost immediately becomes necessary to use consider-
able force to work the helm. This is due to the circumstance that imme-
diately after the screw has made the first backward revolutions the pressure of
water in front on the helm ceases, causing so-called deadwater at the helm ;
whereas an increasing pressure of water on the helm from behind results as
soon as the backward revolutions of the screw begin to gain in rapidity.
The pressure on the rudder from behind, in spite of the vessel's headway, is
presumably to be attributed to the fact that the masses of water thrown
forward by the screw must immediately be replaced, causing a correspondingly
powerful suction, and consequently a current of water acting on the rudder.
The conflict between the contending masses of water is clearly visible
also on the surface on both sides of the ship (particularly on the right

"We beg to hand these results of our observations to the Nautical
Association of this town for distribution amongst other masters, and trust
that they will likewise communicate the result of their experiments with
their respective ships, in order that the data thus collected may be of service
to the seafaring community.

"(Signed) J. SCHUTZ, Master of the s.s. 'Susanne.'
F. WILKE, s.s. 'Marietta.'

C. STREECK, s.s. 'Martha.'

"N.B. The altered operation of the screw being in the main to be
attributed to the fact that. the backward revolutions transmute the pressure
of water on the helm coming from in front to a pressure coming from
behind, it is, when carrying out this manoeuvre, of the greatest importance
that the helm should first be turned after the screw has commenced to re-
volve backwards."

From the 'Shipping and Mercantile Gazette.'

"In connexion with the Board of Trade inquiry recently held at South
Shields, before Mr Stipendiary Yorke, into the collision between tbe ' Tabor '
steamer, of Sunderland, and the brig ' William and Ann,' of Seaham, which
happened in the River Thames, on January 25 last, some experiments have
been made on board the ' Tabor,' at the mouth of the Tyne, on the peculiar
effects of the rudder and screw when reversed while the ship has headway, to
turn the ship's head to starboard of her course. As far as I am aware, the
first public intimation of any similar trials having been made appeared
in the Report of the British Association for the year 1876. The Report
gives an account of certain experiments made on the Clyde by Professor
Osborne Reynolds, Sir William Thomson, Mr James R. Napier, and others.
These experiments were undertaken to verify certain trials made upon models


by Professor Reynolds, which he had brought under the notice of the Asso-
ciation the previous year at Bristol. An able article on the subject, from the
pen of Sir Travers Twiss, appeared in the Nautical Magazine, last year;
so that nautical men should now be aware that they cannot always depend
upon the action of the rudder and screw when reversed while the ship has

"Almost all experienced officers of the navy and merchant service are
doubtless well acquainted with the manoeuvring of screw vessels under steam,
but it is thought that junior officers and others might not have had oppor-
tunities of acquiring information of this kind. There is a marked difference
in the results observed on the Clyde and those noticed at the mouth of the
Tyne, inasmuch as in none of the trials detailed below did the ship's head
swing to port of her course, but invariably to starboard. I would have pre-
ferred to wait until these differences had been either accounted for or
explained by further experiments before placing the facts before your readers ;
but an imperfect account of the trials having appeared in the local news-
papers, it was thought desirable to publish the facts just as they were
obtained, and to reserve for a future letter further remarks, and a de-
scription of a series of similar experiments made by direction of the
Local Marine Board on board the ' Cervin ' steamer, off the Tyne in August
last year.

"While the inquiry in reference to the 'Tabor' collision was pending,
Captain Henderson, the Secretary to the British Shipmasters' and Officers'
Protection Society, suggested to the owner of that ship the importance of
trying some experiments on the effect of the rudder and screw when reversed.
Mr Westall, the owner, immediately gave instructions to his manager to place
the ship, which was then in the Tyne, at the disposal of Mr Yorke. That
gentleman, thinking that it was a point of some interest to mercantile and
nautical men to have the matter settled by actual trial, requested Messrs
Gillie and Tate, the Examiners to the Tyne and Wear Local Marine Boards, to
accompany the ship to sea, and have the experiments carried out under their

"On the 19th inst., the 'Tabor' was unmoored from Shields Harbour, and
at 2 P.M. proceeded to sea in charge of a pilot and Captain Mankin. There
were also on board Rear- Admiral Powell and Captain Nicholas, the Nautical
Assessors ; Mr L. V. Hamel, Solicitor to the Board of Trade ; Captain
Henderson, Secretary to the British Shipmasters' and Officers' Protection
Society, and Mr Roche, their solicitor ; Alderman Peckett, of Sunderland ;
Captain Mail, manager for Mr Westall, the owner, and others.

" The ship was run out to sea three or four miles so as to be out of the
way of passing vessels. The ' Tabor ' is a screw steamer of 520 tons register.


Her length between perpendiculars is 208 feet ; breadth, 27'8 feet ; depth,
14'8 feet. She is propelled by two engines of 90-horse power combined, and
at the time of the trial had in about 200 tons of water ballast. Her draught
of water forward was 6 feet 6 inches, and aft 10 feet 4 inches, she being
nearly in the same trim as she was at the time the collision happened. Her
screw is right-handed, and four blades; diameter, 12 feet; pitch, 17 feet.
The top of the blade was about 2 feet out of the water when the blade was
parallel with the sternpost. The direction of the wind was S. by W. | W. ;
force 4 to 5. The sea was perfectly smooth. The weather being a little hazy,
and the marks upon the land indistinct, a dumb card could not be used
to measure the angles made by the ship's head, but the bridge compass, being
in excellent condition and not sluggish, was used for this purpose. Mr Tate
and Mr Hamel noted the time, the change in the ship's head was observed by
Mr Gillie, who also took down the notes, and Mr Roche noted the time it took
to stop and reverse the engines. The ship's head previous to commencing the
whole of the trials was steadied at W.S.W., and the engines were kept going
full speed ahead, the ship making 8 knots an hour as shown by the patent log.
The fore and main trysails were set during trials 1 and 2 ; there was no
canvas set during trials 3, 4, and 5.

" Trial No. 1 (helm hard a-starboard). At 3.43 P.M., while the ship was
going full speed ahead, the order was given to stop and reverse the engines
to full speed astern, at the same time the helm was put hard to starboard,
both operations being done simultaneously, and completed in 12 seconds from
the time of the order being given. In 40 seconds ship's head fell off to star-
board of W.S.W. 10 degrees ; in 1 minute 30 seconds ship's head fell off to
starboard of W.S.W. 34 degrees ; in 2 minutes 45 seconds ship's head fell off
to starboard of W.S.W. 67 degrees, and ihe ship's way through the water
ahead was completely stopped.

" Trial No. 2 (with helm hard a-port), the ship's head being brought to
W.S.W., going full speed, all other conditions as in trial 1. In 40 seconds
ship's head fell off to starboard of W.S.W. 12 degrees; in 1 minute 30 seconds
ship's head fell off to starboard of W.S.W. 23 degrees ; in 2 minutes 45 seconds
ship's head fell off to starboard of W.S.W. 42 degrees, and the ship's way
through the water ahead completely stopped.

" Trial No. 3 (with helrn amidships), fore and main trysails taken in, all
other conditions as before. In 40 seconds ship's head fell off to starboard of
W.S.W. 15 degrees; in 1 minute 80 seconds ship's head fell off to starboard
of W.S.W. 50 degrees ; in 2 minutes 45 seconds ship's head fell off to star-
board of W.S.W. 79 degrees, and way stopped.

" Trial No. 4 (with helm hard a-starboard), being trial No. 1 repeated with
no canvas set, all other conditions as before. In 40 seconds ship's head fell off


to starboard of W.S.W. 7 degrees ; in 1 minute 30 seconds ship's head fell off
to starboard of W.S.W. 45 degrees; in 2 minutes 45 seconds ship's head fell
off to starboard of W.S.W. 78 degrees, and way stopped.

"Trial No. 5 (with helm hard a-port), being trial No. 2 repeated with no
canvas set. In 40 seconds ship's head fell off to starboard of W.S.W.
16 degrees; in 1 minute 30 seconds ship's head fell off to starboard of
W.S.W. 34 degrees; in 2 minutes 25 seconds ship's head fell off to starboard
of W 7 .S.W. 45 degrees. At this point the engines were stopped by mistake,
but the ship's head appeared to be fixed at W.N.W., and she had very little,
if any, way through the water ahead.

"The practical results of these trials, as far as the 'Tabor' is concerned, is to
show that when she is going full speed ahead in ballast trim, if her engines
are stopped and reversed, her head will go to starboard of the course she is
steering. The helm seems to have very little effect, the results obtained
with the helm hard a-starboard and when it was amidships being very much
alike. With the helm hard a-port the ship's head still went to starboard, but
the angle described was much smaller than that made when the helm was
amidships or a-starboard. I will not at present trespass any further on your
space, but perhaps you will allow me to add that the experiments made with
the ' Cervin ' steamer, with a draught of 22 feet, go a long way to show that,
in ships of her class, similar results to those detailed above will follow under
similar circumstances.

"Local Marine Board, South Shields,

" February 22, 1878."

Remarks by the Committee.

It will be seen that the results in this case are very similar to those ob-
tained in the case of 'North -Western.' The right-handed screw only partially
immersed gave the vessel a strong bias to starboard. But in this case, in
addition to the direct effect of the screw, the effect of the wind, which was of
force 4 or 5, was to bring the vessel round to windward, which happened in
all cases to be to starboard.

In the next vessel reported, the ' Cervin,' it will be seen that the screw
was well immersed, and hence would probably exert no great influence when
reversed to turn the vessel to starboard. At the commencement of all the
trials, however, the wind was blowing with force 5 on the starboard side of
the vessel, and the effect of this would be to cause the vessel, as long as she
had way on, to turn to windward, and this, it will be seen, is what happened ;
in every case the vessel's head turned to windward. Here also the reverse
influence of the rudder was apparent, for the vessel turned faster to starboard
with the helm starboarded than with it ported.


" August 25, 1877.

"S.s. ' Cervin,' of South Shields, length, 287 feet ; breadth, 34 feet ; depth,
24 feet; tonnage, 1913. Propelled by two engines of 180 h.p., combined;
screw right-handed, 4 blades; diameter, 14 feet 9 inches; pitch, 17 feet;
draught of water, forward 21 feet 4 inches, aft 21 feet 9 inches ; top of blade
of screw immersed in water, about 5 feet ; wind, E.N.E. ; force, 5 ; sea

"Trial No. 1 (helm hard a-port). Ship's head N. by W., going full speed
ahead, 9^ knots, the engines were stopped and reversed, and helm put hard
to port ; ship's head came up to N. by E. in 2 minutes, and remained
steady on that point ; way through the water ahead stopped in 3 minutes
40 seconds.

"Trial No. 2 (helm hard a-starboard). Ship's head N. by E., it came
up to N.E. by E., or 45 degrees, in 4 minutes, and way stopped.

" Trial No. 3 (going fast astern, screw started to drive her ahead, helm
a-port). Head N. by W., fell off to N.N.W. in 1 minute 30 seconds.

" Trial No. 4 (with helm a-starboard). Head at N.E., fell off to N.N.E. in
2 minutes.

" Trial No. 5 (full speed ahead, helm amidships). Head N. by W., went
slightly towards west, then back to north, in 3 minutes.

For continuation, see paper 37,



[From the "Philosophical Transactions of the Royal Society."

Part II. 1879.]

(Read February 6, 1879.)

Part I. Experimental Researches on Thermal Transpiration of Gases through
Porous Plates and on the Laws of Transpiration and Impulsion, including
an Experimental Proof that Gas is not a continuous Plenum.

Part II. On an Extension of the Dynamical Theory of Gas, which includes
the Stresses, Tangential and Normal, caused by a Varying Condition of
Gas, and affords an Explanation of the Phenomena of Transpiration and



1. THE motion of gases through minute channels, such as capillary
tubes, porous plugs, and apertures in thin plates, has been the subject of
much attention during the last fifty years. The experimental study of
these motions, principally by Graham *, resulted in the discovery of several
important properties of gases. And it is largely, if not mainly, as affording
an explanation of these properties that the molecular theory has obtained
such general credence.

It does not appear, however, that either the experimental investigations
of these motions or the theoretical explanations of the properties revealed
have hitherto been in any sense complete.

There exists a whole class of very marked phenomena which have escaped
the notice of Graham and subsequent observers ; while several of the most

* Edin. Phil. Tram., 1831; Phil. Trans., 1846 and 1863.
O. R. 17


marked and important facts discovered by Graham have hitherto remained
unconnected by any theory.

2. Amongst the best known of the phenomena is the difference in the
rates at which different gases transpire through minute channels, and the
consequent difference of pressure which ensues when two different gases
initially at the same pressure are separated by a porous plate. It does not
appear, however, that hitherto any attempt has been made to ascertain the
existence of what may be considered a closely analogous phenomenon that
a difference of temperature on the two sides of the plate might cause gas,
without any initial difference of pressure or any difference in chemical con-
stitution, to pass through the plate nor am I aware that such a result from
a difference of temperature has been in any way surmised (see Appendix,
note 1).

I have, however, now ascertained, by experiments which will be described
at length, that a difference of temperature may be a very potent cause of
transpiration through porous plates. So much so that with hydrogen on
both sides of a porous plate, the pressure on one side being that of the
atmosphere, a difference of 160 F. (from 52 to 212) in the temperature on
the two sides of the plate secured a permanent difference in the pressure on
the two sides equal to an inch of mercury ; the higher pressure being on the
hotter side. With different gases and different plates various results were
obtained, which are however, as will be seen, connected by definite laws.

I propose to call the motion of th<& gas caused by a difference of temperature
Thermal Transpiration (see Appencftx, note 2).

3. Again, although Graham found that he obtained not only very
different results but also very different laws of motion with plates of different
coarseness or with plates and capillary tubes*, neither he nor any subsequent
observer appears to have followed up this lead. As regards Graham this
appears to me to be somewhat surprising. For although he may have con-
sidered the mere difference in the results to have been analogous to the
difference found by Poiseuille for liquids, it would seem as though the
difference in the laws of motion which he obtained should have excited his
curiosity ; and then, as he was avowedly of opinion that gas is molecular, he
could hardly have failed to observe that so long as the distance separating
the molecules in the gas bore a fixed relation to the breadth of the openings
in his plates, he should have had the same laws of motion. This view,
however, appears to have escaped him as well as all subsequent observers.
Otherwise it would have been seen that with a simple gas such as hydrogen,
similar results must be obtained so long as the density of the gas is inversely
proportional to the lateral dimensions of the passages through the plates.

* Phil. Trans., 1863.


By experiments, to be described, I have now fully established this law.

1 find that with different plates similar results are obtained when the densities
of the gas with the different plates bear a fixed ratio ; and this is the case
whatever may be the cause of the transpiration, i.e., a difference of temperature
or a difference of pressure (a difference of gas I have not investigated, as it
was obviously unnecessary to do so). Thus with two plates, one of stucco
and the other of meerschaum, similar results of transpiration caused by
pressure were obtained when the densities with the plates were respectively
as 1 to 5'6, both with hydrogen and air and at pressures ranging from 30 to

2 inches of mercury. Also with the same two plates similar results of thermal
transpiration were obtained when the densities were respectively as 1 to 6'5
both for air and hydrogen, and through a range of pressures from 30 to
25 inches of mercury. The discrepancies of 5'6 and 6'5 were in all probability
owing to a slightly altered condition of the plates (see Appendix, note 4).

This correspondence of the results at corresponding densities holds,
although the law of motion changes. Thus with air at 30 inches the law was
the same as that obtained by Graham for stucco plates, while at the smallest
pressures (*25 inch) it was nearly the same as he found for graphite plates or
apertures in thin plates.

4. Having established this law of corresponding results at corresponding
densities, it became apparent that the results obtained with plates of different
coarseness, and with the same plates but different densities of gas, also
followed a definite law. This law, which admits of symbolical expression,
shows that there exists a definite relation between the results obtained, the
lateral dimensions of the passages, and the density of the gas.

This law is important as reconciling results which have hitherto appeared
to be discordant, such as Graham's results with plates of different coarseness,
and as tending to complete the experimental investigation ; but it has another
and a more general importance.

It may not appear at first sight, but on consideration it will be seen that
this law amounts to nothing less than an absolute experimental demonstration
that gas possesses a heterogeneous structure that it is not a continuous
plenum of which each part into which it may be divided has the same
properties as the whole.

It would appear that Graham must have had this proof, so to speak,
under his eyes, and it is strange that both he and subsequent observers have
overlooked it. It seems possible, however, that they were not alive to the
importance of such a demonstration. It is now so generally assumed that
gas does possess molecular structure that the weakness of the evidence on
which the assumption is based and the importance of further proof are points
that are apt to escape notice.



The importance of an experimental demonstration that gas possesses

molecular structure.

5. The idea of molecular gas does not appear to have originated from
the recognition of properties of gas which were inconsistent with the idea of
a continuous plenum, but from a wish to reconcile the properties of gas with
the properties of other substances, or more strictly with some general property
of matter. And the general conviction which may be said to prevail at the
present time is owing to the simplicity of the assumptions on which the
molecular hypothesis is based, and the completeness with which many of the
properties of gases have been shown to follow from the molecular hypothesis.

But it will be readily seen that however simple may be the assumptions
of the kinetic theory, and however completely the properties of gases may be
shown to follow from these assumptions, this is no disproof of the possibility
that gas may be a continuous substance, each elementary portion of which is
endowed with all the properties of the whole, and unless this is disproved
there may exist doubt as to the necessity for the kinetic theory.

Any direct proof, therefore, that gas is not ultimately continuous altogether
alters the position of the molecular hypothesis.

The sufficiency of the demonstration that gas is not structureless.

6. In order to prove that gas is not continuous it is not necessary that
we should be able to perceive the actual structure ; we have only to find some

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