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15


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1


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27



"Ship came to absolute rest in 4 min. 15 sec., her head turned to port
and then 27 to starboard before coming to rest.

" Fourth Experiment.
"In this case the ship was going full speed astern, say about 9 knots,



Time. A. M.


Interval


Ship's Head by
Compass


Head ti
Port


irned to
Starboard


h. m. s.


m. s.


o


o




11 3 11




S. 65*. E.






3 26


15


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3 41


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15


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1


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, 48




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2 45


2 45




2


19*,



28] ON THE EFFECT OF PROPELLERS ON THE STEERING OF VESSELS. 209

when the engines were suddenly reversed to full speed ahead, helm put hard
to port, time and direction of ship's head noted until the ship came to a dead
stop. Sea, wind, and weather as before, viz. most favourable conditions for
these trials.

" Ship came to a dead stop in 2 min. 45 sec., and her head turned 2 to
port in the first 45 seconds and 19^ to starboard in the next 2 minutes.



"Fifth Experiment.

" Making the circle : hard to port : full speed ahead. Lat. 8 50' S.,
long. 153 58' E.

" Ship started full speed from a position of absolute rest, with the helm
hard aport, and at the instant of starting an empty flour barrel was dropped
from the stern to mark the point started from. Sea smooth or nearly so,
between No. 1 and 2 of the Beaufort Scale. Wind very light, about No. 1
to 2.



Time. A.M.


Interval


Ship's Head by
Compass


Arc
turned


h. m. s.


m. s.


o


o


9 27 54




N. 56J W.




28 24


1 30


54 ,


a*


28 54


30


49 ,


5


29 24


30


38 ,


11


29 54


30


28 ,


10


30 24


30


18 ,


10


30 54


30


5 ,


13


31 24


30


N. 6 E.


11


31 54


30


19 ,


13


32 24


30


30


11


32 54


30


43^ ,


13i


33 24


30


58


14j


33 54


30


74 ,


16


34 24


30


89 ,


15


34 54


30


S. 75 E.


16


35 24


30


61 ,


14


35 54


30


46| ,


14A


36 24


30


33 ,


13|


36 54


30


20 ,


13


37 24


30


7i >




37 54


30


4^


12"


38 24


30


16| ,


12


38 54


30


30 ,


13i


39 24


30


45| ,


15*


39 54


30


61 ,


15J


40 24


30


78 ,


171


40 54


30


84 ,


17|


41 24


30


67 ,


17


41 40


16


57 ,


10



" Ship completed the circle in 13 min. 46 sec., and came outside the barrel

O. B. 1 '




210 ON THE EFFECT OF PROPELLERS ON THE STEERING OF VESSELS. [28

(point of starting), about 150 feet, when the barrel was abreast of the taffrail.
That is, we had the barrel on our starboard side when circle was completed.

(Signed) "W. SYMMINGTON,

" Commander s.s. ' Hankow.' "

These experiments need no comment ; they are conclusive as to the truth
and importance of the results previously obtained ; and the Committee thank
Capt. Symmington for his report.

In answer to the request of the Committee, made last year, the Admiralty
have caused experiments to be made as to the effect of reversing the screw
on the steering of H.M.S. ' Speedy,' 273 tons, with a maximum speed of
5 knots an hour. The perusal of the extract of the report on these trials
received by the Committee and appended to this report, shows at once that
the conditions under which the experiments were made were such as to
preclude the possibility of their throwing much light on the subject. The
greatest speed of the vessel was 5 knots, and the effect of the rudder with
the screw reversed was so small, that the vessel, in most instances, turned her
forward end into the wind.

On the receipt of the report of these trials, a letter was written to the
Admiralty, urging them to have experiments made with larger and more
powerful ships, but as yet no further communication has been received.

In accordance with the resolution by which they were appointed, the
Committee have communicated with the Admiralty, the Board of Trade, the
Elder Brethren of the Trinity House, and other Corporations, and copies of
the last year's report were forwarded as soon as they could be obtained ; no
intimation has yet been received of any action being taken by these bodies.

It appears, from an article in the Nautical Magazine of December, that
the last report of the Committee was discussed at the conference of the
Association for the Reform and Codification of the Law of Nations, held last
year at the Ancient House, City of Bremen, when the following resolution
was agreed to:

" It is the opinion of the Conference that the existing international rules
for preventing collisions at sea are not of a satisfactory character, and that it
is desirable that the Governments of the maritime states should take counsel
together with a view to amend these rules and to adapt them more carefully
to the novel exigencies of steam navigation."

The article in the Nautical Magazine was written by Sir Travers Twiss,
and in this and in a subsequent article he discusses the facts established by
the Committee, and their bearing on the question of the alteration of the
rule of the road at sea, pointing out the absolute necessity of modifying
Article 15 of the Amended Board of Trade Steering and Sailing Rules, which
are likely to become law.



28] ON THE EFFECT OF PROPELLERS ON THE STEERING OF VESSELS. 211

These and other notices which have appeared in English and foreign
publications show that the subject has already attracted considerable
attention ; and it is important to notice that in no way have the conclusions
of the Committee been in the smallest degree controverted.

Numerous collisions have occurred during the year, which, to judge from
the law reports, might in many instances have been avoided had the effect of
reversing the screw been known and acted upon ; but it does not appear as if
a consideration of this has influenced any of the judgments given.

The collisions have for the most part been with small ships, and so have
not attracted much attention ; but the loss of the ' Dakota ' was a disaster of
the first magnitude, and if it was not due to the porting of the helm with
the screw reversed it might have been, for as soon as the officers became
aware of their extreme danger (the shore being on their port bow) the helm
was put hard aport and the screw reversed full speed, after which, according
to the evidence of Mr Jones, a pilot on board, the vessel turned to port until
she struck. The evidence offered by the Secretary of the Committee was,
however, rejected by the Commissioner of Wrecks (Mr Rothery), on the
ground that the ship was virtually lost before the screw was reversed. It is
to be noted, however, that the orders to reverse the engines and to port the
helm were avowedly given in the hope of saving the ship, and that had there
been a chance of escape, such action, as shown by all the experiments of the
Committee, must most certainly have reduced it.

APPENDIX.

Extract from Report of Captain of Steam Reserve at Portsmouth, dated

24th January, 1877.

Experiments on the Turning of Screw Ships.

I have the honour to report that, as already reported in my letter, dated
30th September, 1876, to the Admiral Superintendent (through whom I
received the original copy of experiments required), there have been no
opportunities of making experiments on this subject, on account of ships
going out on trial having their time fully occupied, and there have been no
ships in the First Reserve which could be taken out for the purpose.

Observing, however, from the report in the Nautical Magazine referred
to, that the largest vessel of which particulars of trial are given is only
80 tons, I took the 'Speedy,' of 273 tons, out and tried the experiments
required with her: her speed is only about 5 knots; draught of water
7 feet 10 inches; rig one small mast forward; screw right-handed, Griffith's,
two-bladed, diameter 6 feet 1 inch, pitch (j feet. The results are given in
attached sheet.

142



212 ON THE EFFECT OF PROPELLERS ON THE STEERING OF VESSELS. [28

An opportunity also occurred of getting one trial of No. 6 in the
' Euphrates,' while waiting for tide. While going ahead the screw was
stopped and reversed, the helm being kept amidships ; the ship's head came
steadily round to starboard (windward) 12 till head to wind, then fell off to
port, and continued to do so till stern to wind. An experienced pilot
(Mr Harding) who was with me told me beforehand that this would be
the case.

The experiments with the ' Speedy ' were conducted by myself, with the
assistance of Staff-Commander Parker, and Mr Riley, chief gunner of ' Asia '
for Reserve.

I think it may be taken as nearly certain that in all cases of putting the
helm over and reversing the screw at the same time the ship will obey the
helm for a limited time, the amount depending on the way the ship has, her
rig, and the direction of the wind and sea with reference to her course, and
that as she loses her way she will fall off from the wind until she brings it
astern or nearly so. Also, that on reversing the engines with the helm kept
amidships, she will come up head towards the wind, and then fall off before
the wind as she loses her way.

It is going beyond the part of the article marked for my remarks, but
I would venture to express an opinion that it would be highly undesirable
to remove the obligation now imposed on ships "approaching each other,
so as to involve risk of collision," to reverse their engines. If the action of
ships with engines reversed is as I have said above, the reversing not only
reduces the risk of serious damage, by lessening the way of both ships, but
brings them parallel to each other, thereby placing them in a good position
to avoid collision.

I would also submit that it is desirable that attention should be called
to the power of the steering-gear. I think it probable that in large
steamers of great speed, with small crews, and not fitted with steam
steering-gear, the number of men usually kept at the wheel would be
found quite inadequate to get the helm hard over till the speed of the
ship was reduced.

It is worth consideration whether it should not be made obligatory, on
steam-ships over a certain size and speed carrying emigrants or passengers,
to be fitted with steam steering-gear, which I believe is not the case at
present.

I believe a doubt exists with many people whether it is safe and proper
to reverse engines when going at full speed ahead at once to full speed
astern ; this doubt (if it exists) should be removed, and it should be clearly
understood that engines are to stand being suddenly reversed from extreme
speed one way to the opposite extreme.



H.M.S. ' Speedy/ gunboat, 273 tons, 60 horse-power, Griffith's screw, right-
handed, 2-bladed, diameter 6 feet 1 inch, pitch 6 feet. January 24th, 1877.



Trial


Engines


Helm


Wind


Eesult


1.


Going full speed
ahead, suddenly
reversed to full
speed astern.


Hard aport.


Ahead.


Before headway was lost,
head went to starboard
15, lost headway in
1' 15" ; ship's head still
went to starboard with
sternway 180 in 8' 15".


2.


Going full speed
ahead, suddenly
reversed to full
speed astern.


Hard astarboard.


Ahead.


Before headway was lost,
head went to port 20,
lost headway in 50";
with sternway ship's
head went to starboard
88 in 3' 20".


3.


Going full speed
astern, suddenly
reversed to full
speed ahead.


Hard aport.


4 points on
starboard
quarter.


Before sternway was lost,
head went to port 9,
lost sternway in 25" ;
then ship's head went
to starboard.


4.


Going full speed
astern, suddenly
reversed to full
speed ahead.


Hard astarboard.


4 points on
starboard
quarter.


Before sternway was lost,
head went to port, lost
sternway in 1' 22" ;
ship's head went off to
port immediately helm
was put to starboard
101 in 4'.


5.


Full speed ahead
and reversed to
full speed astern.


Amidships.


Starboard
beam.


Ship's head went to star-
board ; lost headway in
I'lO"; still going to
starboard, 90 in 4' 20".


t>.


Full speed ahead.


Amidships.


Starboard
beam.


Ship's head went to star-
board 22J in 5', and
6?i in 9' 32".




Full speed ahead.
(No cause could
be seen for the
ship's head going
opposite ways in
these two trials.)


Amidships.


2 points on
starboard
quarter.


Ship's head went to port
31 in 3' 37", and con-
tinued to go to port till
wind was astern 51 in
9' 4".




Full speed astern.


Put from hard
aport to amid-
ships.





Ship's head went fast to
port.




Full speed astern.


Put from hard
astarboard to
amidships.




Ship's head went to star-
board 66 in 3' 55".



(Signed) CHARLES J. WADDILOVE, Captain,
W. A. PARKER, Staff-Commander,
W. J. RILEY, Chief Gunner,

For continuation see paper 35.



- H.M.S. 'Asia.'



29.



ON THE MANNER IN WHICH RAINDROPS AND HAILSTONES

ARE FORMED.

[From the Sixth Volume of the Third Series of " Memoirs of the Literary
and Philosophical Society of Manchester." Session 1876-77.]

(Read October 31, 1876.)

WHEN the particles of water or ice which constitute a cloud or fog are all
of the same size, and the air in which they are sustained is at rest or is
moving uniformly in one direction, then these particles can have no motion
relatively to each other. The weight of the particles will cause them to
descend through the air with velocities which depend on their diameters ; and
since they are all of the same size, they will all move with the same
velocity.

Under these circumstances, therefore, the particles will not traverse the
spaces which separate them, and there can be no aggregation so as to form
raindrops or hailstones.

If, however, from circumstances to be presently considered, some of the
particles of the cloud or fog attain a larger size than others, these will
descend faster than the others, and will consequently overtake those imme-
diately beneath them ; with these they may combine so as to form still larger
particles, which will move with greater velocity and, more quickly over-
taking the particles in front of them, will add to their size at an increasing
rate.

Under such circumstances, therefore, the cloud would be converted into
rain or hail, according as the particles were water or ice.

The size of the drops from such a cloud would depend simply on the
quantity of water suspended in the space swept through by the drop in its
descent that is to say, on the density and thickness of the cloud below the
point from which the drop started.



29] ON THE FORMATION OF RAINDROPS AND HAILSTONES. 215

My object in this paper is to suggest that this is the actual way in which
raindrops and hailstones are formed. I was first led to this conclusion from
observing closely the structure of ordinary hailstones.

Although to the casual observer hailstones may appear to have no par-
ticular shape except that of more or less imperfect spheres, on closer inspection
they are seen all to partake more or less of a conical form with a rounded
base like a sector of a sphere.




Fig. 1. Perfect Hailstone.

In texture they have the appearance of an aggregation of minute particles
of ice fitting closely together, but without any crystallization such as that
seen in the snowflake although the surface of the cone is striated, the striae
radiating from the vertex.

Such a form and texture as this is exactly what would result if the stones
were formed in the manner described above. When a particle which
ultimately formed the vertex of the cone, started on its downward descent
and encountered other particles on its lower face, they would adhere to it,
however slightly. The mass, therefore, would grow in thickness downwards ;
and as some of the particles would strike the face so close to the edge that
they would overhang, the lower face would continually grow broader, and a
conical form be given to the mass above.

When found on the ground the hailstones are generally imperfect ; and
besides such bruises as may be ascribed to the fall, many of them appear to
have been imperfect before reaching the ground. Such deformities, however,
may be easily accounted for.

The larger stones fall faster than those which are smaller, and conse-
quently may overtake them in their descent ; and then the smaller stones will
stick to the larger and at once deform them. But besides the deformation
caused by the presence of the smaller stone, the effect of the impact may be
to impart a rotary motion to the stone, so that now it will no longer continue
to grow in the same manner as before. Hence we have causes for almost any
irregularities of form in the ordinary hailstone.



216 ON THE MANNER IN WHICH RAINDROPS [29

It appears from the numerous accounts which have been published, that
occasionally hailstones are found whose form is altogether different from that
described above. These, however, are exceptional ; and to whatever causes
they may owe their peculiarities, these causes cannot affect the stones to
which I am referring.

Again, on careful examination, it is seen that the ordinary hailstones are
denser and firmer towards their bases or spherical sides than near the vertex
of the cone, which latter often appeafs to have broken off in the descent.
This also is exactly what would result from the manner of formation described
above.




Fig. 2. Broken Hailstone.

When the particle first starts, it will be moving slowly, and the force with
which the particles impinge upon it will be slight and, consequently, its
texture loose ; as, however, it grows in size and its velocity increases it will
strike the particles it overtakes with greater force, and so drive them into a
more compact mass. If the velocity were sufficient, the particles would strike
with sufficient force to adhere as solid ice ; and this appears to be the case
when the stones become large as large as a walnut, for instance.

An idea of the effect of the suspended particles on being overtaken by the
stone, may be formed from the action of the particles of sand in Mr Tilghman's
sand-blast, used for cutting glass. The two cases are essentially the same, the
only difference being that the hailstone is moving through the air, whereas in
the case of the sand-blast the object which corresponds to the stone is fixed,
and the sand is blown against it.

By this sand-blast the finest particles of sand are made to indent the
hardest material, such as quartz or hard steel ; so that the actual intensity of
the pressure between the surface of the particles of sand and that of the object
they strike must be enormous. And yet the velocity of the blast is not so
much greater than that at which a good-sized hailstone descends. It is easy
to conceive, therefore, that the force of the impact of the suspended particles
of ice, if not much below the temperature of freezing, on a large hailstone,
would drive them together so as to form solid ice ; for the effect of squeezing
two particles of ice together is to cause them to thaw at the surface of



29] AND HAILSTONES ARE FORMED. 21 7

contact, and as soon as the pressure is relieved they freeze again ; and hence
their adhesion.

Nor does there appear to be any other way in which these ordinary hail-
stones can be formed. They are clearly not raindrops frozen, or they would
be somewhat transparent ; neither are they aggregations of snow crystals.
Nor can they be formed by the condensation and refrigeration of vapour on a
nucleus of ice ; for there is no way of getting rid of the heat which must be
developed by such a process: the heat developed by the condensation of
vapour one-seventh of the weight of the stone would be sufficient to thaw the
entire stone.

The hailstones are clearly aggregations of small frozen particles such as
those which form a cloud. Nor is it possible that they can have been drawn
together by some electrical attraction ; for whatever such attraction we can
conceive, it will not explain the conical shape of the stones or their increase
in density towards their thicker sides. These clearly show that the particles
have aggregated from one direction, and with an increasing force as the size
of the stone has increased.

It appears as though it might be possible to make artificial hailstones.
If a stream of frozen fog were driven against any small object, then the frozen
particles should accumulate on the object in a mass resembling a hailstone. Not
seeing my way to obtain such a stream of frozen fog, I thought it might be
worth while to try the effect of blowing very finely powdered plaster of Paris.
I therefore introduced a stream of this material into a jet of steam issuing
freely into the air (which I hoped would moisten the powdered plaster
sufficiently to cause it to set firmly in whatever form it collected into). The
jet was directed against a splinter of wood.

In this way I obtained masses of plaster very closely resembling hail-
stones. They were all more or less conical, with their bases facing the jet.
But as might be expected, the angles of the cones were all smaller than those
of the hailstones. Two of their figures are shown in the sketches annexed
(p. 218).

The stria3 were strongly marked, and exactly resembled those of the hail-
stone. The bases also were rounded. They were somewhat steeper than
those of the hailstone; but this was clearly due to the want of sufficient
cohesive power on the part of the plaster : it was not sufficiently wet. Owing
to this cause also it was not possible to preserve the lumps when they
were formed, as the least shake caused them to tumble in pieces.

I also tried a jet of the vapour of naphthaline, which at ordinary tem-
peratures is solid, driven by means of a cross blast of air against a small
object ; and in this way I obtained masses closely resembling hailstones : but



218 ON THE MANNER IN WHICH RAINDROPS [29

these also were too fragile to bear moving. At ordinary temperatures the
powdered naphthaline does not adhere like ice when pressed into a lump. No
doubt at very low temperatures ice would behave in the same way ; that is to
say, the particles would not adhere from the force of impact. Hence it would
seem probable that for hailstones to be formed the temperature of the cloud
must not be much below freezing-point.





Figs. 3 and 4. Imitations in Plaster of Paris.

That the temperature of the cloud exercises great influence on the
character of the hailstones cannot be doubted ; and if, as has been sug-
gested by M. L. Dufour, the particles will sometimes remain fluid, even when
the temperature is as low as F., it is clear that as they are swept up by a
falling stone they may freeze into homogeneous ice, either in a laminated
or crystalline form. Upon these questions, however, I do not wish to enter,
as they have no bearing on the question as to the manner in which the mass
of the stone is accumulated ; and I only mention them to show that, if there
are unexplained peculiarities, there are also causes the effects of which have
not as yet been fully considered.

This view of the manner in which hailstones are formed at once suggests



29] AND HAILSTONES AKE FORMED. 219

that raindrops may be formed in the same way ; uor does there appear, on
further consideration, to be any reason to suppose that such is not the case.

Of course a raindrop shows none of the structural peculiarities of the hail-
stone ; and consequently we have not the same evidence of the manner in
which raindrops are formed; but the explanation is sufficient, and there is
apparently no other.

Raindrops cannot possibly have grown to the size with which they reach
the earth by the condensation of the vapour of the air which they pass
through, for the same simple reason as that just stated for hailstones, namely
that there is no way in which the heat developed by condensation can be got



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