NA TURE
630
altogether but with torrents transporting 5 per cent, and of sohd matter, and with the discharge of sewage, it is beheved 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 verj- abstruse question but I believe that the true answer is given in the brief statement that water
more
—
rolls rather
so
than
slides.
Were it not so, a ship with a foul bottom could not be much retarded when passing through the water as
For example, supposing there experience shows she is. 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 in such a case the coating of pitch on the first ship displacement and the lines arc 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, in the 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 sliding 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 beheved, on a body passing through water, or water flowing down a channel, the particles 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 but any one in a gale of wind, going behind a high lines square block of building, would very soon discover that, for if he went a few in air, such is not the case 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 to leeward where the currents again converged, while beyond this the storm raged with the same violence (Every boatman knows what it is as at any other point. the sea may be to get under the lee of a very high island 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 Within the space of air whirling about in all directions.
April
21,
1870
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 neither could the several lines B B', C C, D D', E E' 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 probably depending on the velocity showing that of there is neither a sliding motion nor a parallelism in the direction of the lines of current.* T. Login
A
THE CLIMATE OF IRELAND no chapter more interestthe science of nature there IN ing than that which treats of Physical Geography, is
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 It has been stated in a theme of no slight importance. the House of Commons as a proof of the retrograde condition 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 Hence Irish allowing them to quit the beaten path. 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 It is commonly stated that Ireland has of Great Britain. It has undoubtedly a humid atmoa very wet climate. sphere, owing, perhaps, in some measure, to a great extent But the total quantity of rain that of undrained surface. falls in Ireland, little, if at all, exceeds the rainfall of England. In its distribution through the year, however,
The vicinity of the differs much from the latter. 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 it
coasts,
though seldom
free
from wind and drizzling
rain,
Vegetation remains in never experience severe cold. 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 When the sun is highest in the little or no dry summer. meridian, there is a constantly clouded sky and no sunshine. Rain begins to fall in June. The rainfall of July In August the rain begins to is the heaviest in the year. 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
From
rain in excess,
and too
little
sunshine.
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.
ABCDE
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.
this
it
will
Now, to cultivators of the cereals the success of this proThe corn harvest in cess is of the utmost importance. 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 maj^ throw light on the proper length of the after portion of ships intended for different speeds.
