To maintain the vessel upright after she has grounded on the keel blocks, and before the bilge blocks are hauled, wale shores are used, one end resting against the ship's side, the other against the dock's side, wedges being used to set them taut. These shores are prepared of the desired length and placed in the vicinity of their final location by means of information obtained from the docking plan. Certain marks are also made on the coping which will accurately locate the ship's position in the dock, in order to ensure that as the water is pumped out, the under-water hull shall exactly coincide at the proper time with the various blocks and shores which have been made ready to receive it. Plate 147 shows the plan which would be prepared for a particular dock after receiving the docking plan of the vessel which was to be placed therein.

These preparations being completed, water is admitted, the caisson is floated and removed, and the dock is then ready to receive the ship. After the ship's bow has safely entered the mouth of the dock, the responsibility for her safety rests upon the dock master; the methods of securing this safe entrance are considered elsewhere. The dock master then hauls the ship into the dock until certain definite objects near her bow and stern coincide with the marks which he has laid out upon the dock coping in accordance with the docking plan. The caisson is then placed in position, the pumps which empty the dock are started; their operation between this time and the time of the complete emptying of the dock being controlled by the judgment of the dock.

master.

In the meantime the necessary arrangements have been made to ensure the ship being safely centered in the fore and aft and athwartship directions, and the wale shores have been floated and placed approximately in their proper positions: during this period, by the use of sighting battens or other means, the variations of the ship from the upright are finally determined, and the necessary measures are taken to correct any listing to starboard or to port.

Under ordinary circumstances, the ship's keel first touches on the keel blocks aft, and with a ship having a large amount of drag, special precautions are necessary to prevent listing, because, under these circumstances, stability is lessened very rapidly. The grounding of the keel aft upon the keel blocks is indicated. in various ways, but before this occurs, the dock master has ar

ranged the wale shores so that any inclination towards listing or twisting shall be prevented as far as possible. The entire line of wale shores is not set up tightly until the forward portion of the keel is bearing upon the keel blocks prepared to receive it. As the water level within the dock becomes still lower, the bilge blocks are hauled, and the ship during the remaining period of her stay in dock is supported by the keel blocks, bilge blocks, and shores.

After the dock is emptied, the ship's bottom is thoroughly cleaned and careful examination is made of the entire bottom as regards fouling, corrosion and damage. All outboard valves, propeller struts, propellers, shaft bearings, rudder pintals and gudgeons, strake edges, butts, etc., are carefully examined. It is usually necessary to re-grind the underwater valves, and to repack the stuffing boxes of valves and of the rudder; if the plating butts show lines of rust, they should be re-calked; if the rivet heads show serious corrosion, the rivets should be removed and new ones driven; if there is serious corrosion or pitting, the plating should be thoroughly cleaned and brushed before re-painting; in cleaning the bottom from fouling substances, care should be taken that the paint underneath is disturbed as little as possible; zinc rings and zinc plates at the openings of outboard valves, and in the vicinity of the propellers, should be renewed if their corrosion shows galvanic action. The bottom is then given fresh coats of anti-corrosive and anti-fouling compositions, which should be applied whenever possible, on dry surfaces.

If special repairs have been anticipated or are found to be necessary after the dock is empty, the necessary action is taken immediately, because the length of time a ship remains in dock must be reduced to the minimum.

The particular precautions which must be taken in docking ships which are not in ordinary condition, must depend upon the judgment of the dock master.

During the period that a ship is in dock, no change of any kind in the distribution of her weights should be made without the knowledge and consent of the dock master, because the ship when being floated might suddenly change her trim so as to cause serious damage to herself or to the dock.

The painting of the bottom, and all under-water repairs being completed, a time for flooding the dock is agreed upon by the commanding officer and the dock master. The former stations

men at the outboard valves and elsewhere, as he deems proper to ensure that water does not enter the ship, and the latter stations men at the various shores and lines and elsewhere, to prevent as far as possible, any injury to dock or ship, from a change of weights or an unexpected alteration in tide or wind.

The water in the dock enters continuously under the dock master's control. When it has risen to a sufficient height, the bow ordinarily first lifts from the keel blocks, and shortly afterwards the stern. If there has been any material change of weights while the ship has been in dock, she will suddenly and violently take a list to starboard or to port, with consequent damage to herself and the dock.

The ship being safely afloat, the water is allowed free entrance until the level within the dock coincides with that outside, after which time the caisson is floated as quickly as possible, then removed, and the ship is floated out of dock.

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CHAPTER XIX.

WEATHER AND THE LAWS OF STORMS.

§ I. WEATHER, WINDS, CLOUDS, RAINFALL.

The subject of weather may conveniently be studied in its general features by reference to a map showing normal or average conditions at different seasons of the year. Plate 148 gives such a map, showing normal winds, barometer pressures, and temperatures, for January and July, throughout the world with the exception of the extreme polar regions. The curves in black are "isobars" or lines of equal barometric pressure. It will be seen that these lines are, in the main, closed curves about areas of high or low barometer;-the distinction between a "High" and a "Low," as these areas are called, depending not upon the actual height of the barometer, but upon the way in which the wind circulates about the area; or, more accurately, upon whether the characteristic pressure is the result of descending or of ascending currents. It is clear that a high pressure is the natural accompaniment of descending currents of heavy air, and vice-versa; but whereas a barometric height of 30 inches may indicate a rising current at one place and time-which would make it a Low;-it may indicate a descending current at another place and time, and may thus be a High. It tends to clearness, therefore, to use, instead of "high" and "low," the terms "anti-cyclone" and "cyclone, which define the wind circulation instead of the pressure resulting from this circulation. As thus used, the term cyclone has no necessary connection with a gale. It defines simply a condition of affairs in which an ascending current of air at a more or less clearly marked central area is surrounded by an inflowing spiral whirl which may or may not take on the concentrated and intense character with which we commonly associate the term cyclone. Similarly "anti-cyclone" defines a condition of affairs in which a descending current of air develops into an outward flowing spiral whirl with a motion of rotation opposed to that of the cyclone (Plate 149). Both of these phenomena will be more fully described hereafter.

The relative positions of the warm red and cold blue tints

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