630 
NATURE 
[April 21, 1870 
altogether; but with torrents transporting 5 per cent. and 
more of solid matter, and with the discharge of sewage, it 
is believed that the retarding power is quite appreciable. 
The whole question is no doubt a very complicated one; yet 
by a set of careful experiments, conducted with a view to 
discover this adhesive power of water, it appears highly 
probable that an important step would be gained, towards 
the solution of some other difficult but important problems. 
The next point to consider is—How does water travel ? 
This also is a very abstruse question ; but I believe that 
the true answer is given in the brief statement that water 
rolls rather than s/zdes. 
Were it not so, a ship with a foul bottom could not be 
so much retarded when passing through the water as 
experience shows she is. For example, supposing there 
are two ships in every respect the same, only that the first 
is covered with a coating of clean pitch a quarter of an inch 
thick all over her bottom to above her water line ; and that 
the second, in place of the pitch, has got all her bottom 
covered with marine animals and weeds, so that when this 
second ship is passing rapidly through the water, none of 
the sea-weeds or marine animals extend more than this } 
inch beyond the ship’s sides, which is the thickness of the 
coating of pitch on the first ship: in such a case the 
displacement and the lines are exactly the same, but it is 
hardly necessary to ask any sailor which of the two ships, 
with the same wind and sails, would pass most rapidly 
through the water, and, inthe case of two steamers, the 
extra resistance caused by the foul bottom could be easily 
measured in extra horse power required to force the foul 
vessel through the water at a speed equal to the other.* 
If the motion of the water was a s/iding one only, the 
speed in both cases would be the same with the same 
power, for the resistance would be simply the separation 
of the two films of water, the one in contact with the ship’s 
sides and the other with the surrounding sea ; and these, 
in both cases, would be identical, the displacement being 
the same. If, however, as is believed, on a body passing 
through water, or water flowing down a channel, the par- 
ticles of water are set in motion in a revolving direction, 
the convolutions increasing directly in proportion to the 
wetted surface, then by this hypothesis some assignable 
reason for this retarding of the foul-bottomed ship can 
be given. 
If the particles slid over each other rather than rolled, 
they would, so to speak, pass each other in parallel straight 
lines ; but any one in a gale of wind, going behind a high 
square block of building, would very soon discover that, 
in air, such is not the case: for if he went a few 
yards away in the direction the wind was blowing, he 
would soon discover that the building no longer afforded 
any protection from the blast, but that there was some 
certain point toleeward where the currents again converged, 
while beyond this the storm raged with the same violence 
as at any other point. (Every boatman knows what it is 
to get under the lee of a very high island ; the sea may be 
smoother, but the sudden gusts of wind are often more 
dangerous than when exposed to the full force of the gale.) 
Immediately in rear of the wall itself he would find eddies 
of air whirling about in all directions. Within the space 
AB CDE there would be a partial protection from the 
storm, and instead of the wind being in the direction 
* Possibly by the introduction of an elastic medium, such as air, between 
the ship’s bottom and the water, the skin friction may be reduced, as it may, 
in a measure, reduce this rotatory action. 
shown by the arrow, there would be whirling eddies 
within this space, which could not exist were the air to 
pass off in straight lines as represented by the dotted 
lines B B’, C C’, D D’, E E’; neither could the several 
currents of air converge at the point A, which it is well 
known is always the case. : 
In the same manner any obstruction placed in a 
stream of water, causes eddies in rear of it; that is to say, 
the water does not pass on in straight lines, but within this 
space it goes revolving about in all directions, the distance 
of A probably depending on the velocity : showing that 
there is neither a sliding motion nor a parallelism in the 
direction of the lines of current.* 
T. LOGIN 
THE CLIMATE OF [IRELAND 
N the science of nature there is no chapter more interest- 
ing than that which treats of Physical Geography, 
which, properly understood, means the account of physical 
phenomena as they are modified by geographical position ; 
and at the present moment the physical geography of 
Green Erin, or it speculiarities of soil and climate, presents 
a theme of no slight importance. It has been stated in 
the House of Commons as a proof of the retrograde con- 
dition of Ireland, that its production of cereals has of late 
years diminished, while its pasture lands have increased. 
To this it ought to have been answered that the decreased 
cultivation of cereals, and of wheat in particular, was a 
proof of improved knowledge. Years ago, at the meeting 
of the British Association in Cork, a communication was 
read, pointing out that agriculturists in general are 
governed wholly by example, their scanty science not 
allowing them to quit the beaten path. Hence Irish 
farmers, when they aim at improvement, endeavour to 
imitate the farming of Norfolk or the Lothians, and in so 
doing fail miserably, owing to the wide difference between 
the climates of the western isle and of the eastern side 
of Great Britain. It is commonly stated that Ireland has 
avery wet climate. It has undoubtedly a humid atmo- 
sphere, owing, perhaps, in some measure, to a great extent 
of undrained surface. But the total quantity of rain that 
falls in Ireland, little, if at all, exceeds the rainfall of 
England. In its distribution through the year, however, 
it differs much from the latter. The vicinity of the 
Atlantic gives Ireland in the highest degree an oceanic 
and, to some extent, an equatorial climate. Winter in the 
Green Isle is extremely mild. The southern and western 
coasts, though seldom free from wind and drizzling rain, 
never experience severe cold. Vegetation remains in 
mid-winter brilliantly green and undepressed. As spring 
advances, everything seems to flourish ; crops of all kinds 
promise abundance, and already, in May, harvest seems 
to be close at hand. But now the scene changes. There is 
little orno dry summer. When the sun is highest in the 
meridian, there is a constantly clouded sky and no sun- 
shine. Rain begins to fallin June. The rainfall of July 
is the heaviest in the year. In August the rain begins to 
abate ; but clear skies and bright sunshine cannot be 
reckoned on till September, when the shortened days and 
the sun’s declination have much reduced the solar heat. 
The crops in the meantime, arrested in their progress, are 
not the better for two months’ slumbering under the clouds 
They have summer rain in excess, and too little sunshine. 
From this it will be seen that the character of the Irish 
climate is, that under it everything grows well, but that 
the process of ripening is painfully slow and uncertain. 
Now, to cultivators of the cereals the success of this pro- 
cess is of the utmost importance. The corn harvest in 
Ireland falls late in the year, in September and October, 
when the days are short and nocturnal frosts not unfre- 
* By an experimental study of this subject, it may be discovered how far 
these eddies extend with different velocities, which may throw light on the 
proper length of the after portion of ships intended for different speeds, 
