illustrated by the plumb-line xy let fall from the hand. Usually, however, it is not necessary to resort to this scheme; in most cases there will be found three marks as at c, b, d, Fig. 2, on the wristplate bracket and another mark, a on the hub of the wristplate. These will enable us to do our centering. The marks are so located that a is opposite b when the wristplate is at its center of motion. At the two extremes of motion a is opposite either c or d.

It may be well, however, to test these marks, or rather to see that the eccentric

Having tested the marks to our satisfaction, we will temporarily secure the wristplate w at its center of motion.

Upon removing the back bonnets, or caps, from the ends of the valve chambers so that the rear ends of the valves are exposed, we find a mark on each face of the valve ports, showing the location and width of the port openings in relation to the cylinder. Upon the ends of the valves are marks which are in line with the opening edges of the valves. See Figs. 3 and 4. Possibly, in some of the older types of engines, these may be missing;

and carrier rods have proper adjustment relative to the motion of the wristplate. To do this we rotate the eccentric p on its shaft, having the eccentric rod o connected and the carrier rod c hooked over on the wristplate; then notice whether or not the carrier arm is equidistant, in its extreme travel each way, from the plumb-line ay let fall through the center of its pin. If it is not, we will make it so by adjusting the length of the eccentric rod o. Then we see if the mark on the wristplate hub agrees with those on the bracket at full throw each way; if not, the remedy is to change the length of the carrier rod c until there is perfect agreement.

FIG. 2.

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in such a case, the valves will have to be removed to locate the port openings and the opening edges of the valves. Consulting the above table, we find the lap for the steam valves, and the opening to be given the exhaust valves, for our particular case. Then, by lengthening or shortening the rods leading from the wristplate to the valve arms, we bring the opening edges of the valves to positions corresponding with our predetermined lap or opening. While making these adjustments, it is, of course, essential that the steam latch shall be hooked on the stud. If a record is kept of how much the valve moves at one turn of the

adjusting nut on the rod, future adjustments may be made without necessitating the removal of the bonnets.

All the valves are now supposed to be in their proper positions when the wristplate is at its center of movements. The section shown in Fig. 5 gives this position, except that a' is supposed to be unhooked and to stand in a position of full closure. The next thing in order is to locate the eccentric at the proper angle ahead of the crank to give sufficient lead. First, we will set the engine exactly on its center; and with the carrier rod hooked on the wristplate stud, revolve the eccentric on the shaft, in the direction in which the engine is to run, until it is at an angle greater than 90° ahead of the crank, or until the steam valve on the end at which the piston stands is just beginning to open, say, of an inch open. In this position the eccentric must be secured to the shaft. Then we turn the engine to the other center and see if the steam valve on the other end has the same amount of opening as the other had. It should and will have the same amount, if all our adjustments have been carefully made.

We will now turn our attention to the governing apparatus. The function of a governor is to act in accord with every variation in load, and to so limit the quantity of steam admitted to the cylinder as to overcome the resistance of the load, and to maintain a uniform speed of rotation of the engine shaft. For the purpose of adjustment, we block the governor so that the balls stand in the position they would assume at normal speed (about mid position), and fasten the reach-rod lever m at right angles to a line M N, Fig. 1, midway between the reach rods. Now we will turn the engine to the point at which cut-off should occur (usually about stroke), and adjust the reach rod for that end, so the valve will trip at that point. The valve and the reach rod for the other end of the cylinder must be treated in a like manner. To determine the point of stroke, we mark the length of stroke on the crosshead guides and measure off of this from each end. After a few trials, partially rotating the engine back and forth, at the same time making careful adjustments of the reach rods, we can make it cut off at exactly similar points for each end. It is well, now, to lower the governor to the lowest position, and observe that the cut-off mechanism does not work, but allows steam to be taken during the full stroke of the piston.

Care must be taken, in making adjustments

THE

THE art of conducting a ship, with safety and despatch, from one place to another, across the trackless ocean, and, more particularly, the determination from time to time, and at any time, of the position of the ship, is called navigation. As a practical art, navigation is employed by many; but, even among those who make daily use of it, there are comparatively few who understand its fundamental principles; and this, in spite of the fact that its most conspicuous feature is simplicity itself.

History tells us that navigation was first practiced by the citizens of Tyre. These energetic people did much to cultivate foreign commerce, and made their city the great emporium for the trade of Europe and the East. As time went on, and the merchant fleets from Tyre spread through the Mediterranean, many colonies were founded, the most famous of which-Carthage-soon not only equalled, but surpassed, in importance, Tyre itself. From the sixth to the fifth century, B. C., the Greeks made considerable progress in the art of navigation, and during the Peloponnesian war the Athenians displayed remarkable skill in naval tactics. It would be difficult to enumerate the successive steps by which the art of navigation has been brought to its present high state of perfection; but as conspicuous points in its history, the following will perhaps suffice: the invention of Mercator's chart, 1569; the formation, by Wright, of tables of meridional parts, 1597; Davis's quadrant, about 1600; the application, by Gunter, of logarithms to nautical calculations, 1620; the introduction of middle-latitude sailing, 1623; the measure of a degree on the meridian, by Norwood, 1631. Hadley's quadrant, a century later, rendered observations easier and more accurate; while Harrison's chronometer, 1764, made the computation of longitude a matter of comparatively small difficulty.

Navigation is divided into two branches -dead reckoning and nautical astronomy. These two methods, while absolutely independent of each other, are in practice gener

ally carried on together, one serving as a check upon the other.

When a vessel is about to leave one port for another, she is usually conducted out of the harbor by a pilot. This pilot lays his courses by the ranges with which long acquaintance has made him familiar. Arrived at the limit of his field of usefulness, that is, at a point where his local knowledge is of no further value, he leaves the vessel, and from there the navigating officer assumes all responsibility. While in sight of land, the navigator steers his ship by his charts and by the lead, assisted during the day by landmarks and buoys, and at night by lights. It is his duty, while off the coast, to keep the lead going quite frequently, no matter how fine and clear the weather may happen to be, or how confident he may feel as to the exact position of his vessel. When, finally, he is about to lose sight of land, a last position, called the point of departure is determined, which point then serves as a base for future operations. The problems involved in a long voyage are many and various, and often the commanding officer's skill and courage are taxed to the utmost. However, the ship's position is always ascertainable, either by dead reckoning or by observations of celestial bodies. Speed and direction are two very important factors to the navigator. The first is found by the log, and the second is indicated by the compass, from which is read the angle between the magnetic meridian and the ship's keel; this angle is generally known as the course. The log, the unit of which is the knot, predicates the number of nautical miles (6,082 feet) traversed per hour; but the log is not an accurate instrument, nor is it possible, in a sailing vessel, to take as many readings as the frequent changes in the speed of the vessel would call for. Again, the course of a vessel is not always the one read on the compass, because the action of the wind on the sails produces a certain side-push, forming an angle with the keel, and resulting in a falling off from the true course. This angle is known as drift, and the amount of it is usually ascertained

from the wake of the ship by a backsight of the compass, and combined with the magnetic variation, in order to obtain the true course. Allowance must also be made for the influence of local, tidal, or ocean currents, the strengths of which are either indicated on the charts or known by experience. Currents always seriously interfere with the calculations of a navigator, and too much attention can never be paid to them. The course and speed of a vessel are noted at the end of each hour on a tally, and at the close of each watch (every fourth hour) it is transferred to the log book. Every day at noon-or oftener, if deemed advisable-the reckoning is cast up, and the position of the vessel is marked on the chart, taking as a base the last ascertained position. All navigation by dead reckoning is checked at least once a day, and as often and in as many different ways as can be accomplished by observations of celestial bodies.

Dead reckoning is comparatively simple, is easy to practice, and may be relied upon for short distances; but there are several causes which render it untrustworthy during a voyage of any length. The various strengths of the different currents which the ship may happen to enter will materially upset the accuracy of it. For instance, a ship bound west from some English port, and steering a due westerly course, is caught by an unknown current, running north with a velocity of, say, five miles per hour. Should the navigator, under such circumstances, depend entirely upon dead reckoning, the probability is that he would be wrecked somewhere on the coast of Newfoundland, at a time when he thought himself several hundreds of miles away from the nearest shore, since there would be nothing whatever in the appearance of the sea to indicate the existence of any current. Again, a ship may run into a series of furious, changeable gales, that toss it about for days and leave the dead reckoning in a state of entire muddle and confusion. At such times the ship's safety depends solely upon that branch of navigation defined as nautical astronomy.

and HH' the stationed at m.

horizon of the observer, Now, the latitude of a place

on the earth's surface is defined as the angular distance north or south of the equator, measured on the meridian running through the place; and, since the meridians all intersect at the poles p and p', the latitude of the place m is the arc me on the meridian p me p'. To obtain a value of this arc me (which is equivalent to the arc Z E on the celestial meridian), assume the sun to be situated at S. Then, SZ is its zenithdistance, S E' its declination, and SH' its meridian altitude. Hence, the angle Zo E is equal to ZS + SE', or, in other words, the latitude of m is equal to zenith-distance + declination. Zenith distance is the complement of the altitude, and is either north or south-north, when the sun is bearing south, and south when the sun is bearing north. The declination of the sun is found in the

"Nautical Almanac" for every day of the year, and is either north or south, its maximum value being 234°.

Again, should the sun be at S', and its declination E'S' south, ZS' being the zenith distance, the angle Zo E' is equal to ZS' E'S', or the latitude in this case equal to zenith distance declination. Hence, the latitude of any place is equal to the sum of zenith distance and declination, when both have the same name, and to the difference when they are of contrary names, the latitude then having the same name as the greater of the two. A meridian altitude of the sun is always taken at noon, or at the moment when the sun has reached its point of culmination; but to the observed altitude must always be applied certain corrections, such as 'dip," "refraction," "semi-diameter,"

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