The above changes are not excessive for chronometers slightly canted, and the cant is what would take place were the screws in one of the adjusting slots of the gimbals to become loose, allowing the bowl to slide to one side or the other.

At some time during the trial, after the chronometers have passed through the temperature test, and their curves have been determined, they are rated for one term of seven days each with the XII of their faces North, South, East, and West, successively, as a test for polarity; and should any evidences be found, they are again put through the different positions to make sure that the irregularities are not merely coincidences. Chronometers having actual polarity are at once rejected.

The chronometers are relatively placed after trial by the trial number, which is: Trial number(69°0)+1000 + 10 v2, in which 69° Fahr. is the temperature at which the chronometers should be compensated to have their fastest rates, and it is obtained by taking for a number of years the mean of the chronometer boxes of several vessels of the navy serving on different stations. z is the temperature-constant, and the arithmetical mean of the five greatest variations of the mean rates from the curve as determined by formula (5).

(69°-0) should in no case exceed 10°, except in chronometers compensated for special purposes, and it should be allowed to approach 10° only when z is very small, making the curve nearly a straight line. z should in no case exceed .006, and should be allowed to approach it only when (69° — 0) is very small, thus placing the large variations of rate, due to temperature, in the extremes to which the chronometers would seldom be subjected. v should in no case exceed 0.50s, making due allowance for coefficient of time, especially in new chronometors.

Chronometers failing to pass the required trial are returned to their makers, and rejected, if new and for purchase; but if old ones having been cleaned and repaired, they are to be recompensated free of charge and returned for a new trial.

Chronometers for service are selected with a view to the temperature in which they are to be used. Those for warm climates should have their point of compensation highest, and those for cold climates the reverse. Those to be used in both cold and warm climates should have their temperature-constant small; and in all cases the

smaller the value of z, the nearer the curve will approach a straight line and the better will be the chronometer, all other things being equal. On being issued, chronometers are transported by hand direct from the Observatory to their destination while still running, and are handled with the greatest care. For transportation they are taken from their gimbals, wrapped closely in paper and placed level in a basket of cotton, cushioned around the edges and from each other, but not tightly packed. They are delivered to the navigation officer of the navy yard or of the vessel, as the case may be, when the supervision of the Observatory ceases until their return. Their boxes are enclosed in the transporting cases, neatly crated and sent as freight.

In transportation the principal things to be guarded against are circular motion and placing them in any other position than a level one. If their error and rates are to be carried on in transportation, a maximum and mimimum thermometer should be packed in the basket as near as possible to the chronometers to obtain their mean temperature en route.

NAVAL INSTITUTE, ANNAPOLIS, MD.

FEBRUARY, 1884.

CHARTS AND CHART MAKING.

BY LIEUTENANT JOHN E. PILLSBURY, U. S. N.

The question of aids to safety in navigation is of great importance to a very large portion of mankind. The merchant who is interested in the vessel or the cargo ventures his capital; the passengers, who as a rule know the least of the dangers to be overcome, and who are obliged to have faith in the ability of the officers in charge and in their appliances, are risking their lives; and the officers, on whom the whole responsibility rests for the safety of the property under their charge, as well as for the lives of those on board, are staking their reputations as well as their lives every time they get underway.

Without a chart of some sort the question of navigation would be reduced to a matter of considerable difficulty and great hazard; and in this day of high speed, quick sales for early cargoes, increase of pleasure-travelling, and efforts for the repeal of compulsory pilot laws, it is imperative that the piece of paper on which are depicted the shore-line and contour of the bottom shall be as nearly true to nature as it is possible to make it, and should show everything that can in any way assist in bringing the vessel safely into port.

As a rule, from the beginning of chart-making, governments have assisted and fostered work of this kind; but it is only in comparatively recent times that governments have engaged in a systematic collection of data, and an embodiment of the data on well-digested schemes of charts embracing the whole world.

There may be some present who are not wholly familiar with the methods which are at present in use in the Coast Survey in preparing the charts for issue from the beginning. At the risk of repeating a story familiar to some, I would like to relate the various steps in chart-making for the benefit of the others. The primary triangula

tion of our own coast has been practically finished, and the secondary triangulation nearly so. As the secondary triangulation is carried on, the topography is executed according to schemes drawn up by the Superintendent. The Hydrographic Inspector, in consultation with the Superintendent, receives directions as to the localities of the proposed hydrographic work, and then lays out the scheme of projections for each party according to the nature of the work. The draughtsmen in the office prepare the projections according to this scheme, and the chiefs of the hydrographic parties eventually receive them, together with descriptions of triangulation points, bench-marks and detailed instructions from the Hydrographic Inspector.

Upon the conclusion of the survey the projections are returned to the office, showing the fixed positions on all the lines of soundings, appropriately lettered and numbered, together with sounding-books, showing the soundings and times, and the angles taken with them; anglebooks, showing the angles used to cut in hydrographic signals from the triangulation points; tide-books, showing the reading of the tidegauge during the progress of the survey, and, lastly, sailing directions, or a description of points of interest developed during the work. Draughtsmen attached to the office of the Hydrographic Inspector then verify the positions, plot the soundings, and, after the finished sheet is registered, the draughtsmen of the drawing division make a reduction of it for the engraver, on the exact scale to be used for the finished chart, which is verified by the Hydrographic Inspector by comparison with the original. In the meantime a project has been decided upon, showing the limits of the proposed chart; the topography has been reduced and a plate has been prepared ready for the engraver. As a rule, engravers have two plates on hand at once, so that while one is being proofed, or is waiting for additional work, or is delayed from any cause, the other may be taken up and no time be lost. As the plate approaches completion in hydrography and topography, the magnetic variation is ascertained for the probable time of issue; the light-house table prepared, and the "aids to navigation" are obtained from the Light House Board, and the title and notes decided upon.

When the plate is finished it goes to the electrotype department to be duplicated. One plate will stand only about 1200 or 1500 impressions, and in order to save cost of reproduction by engraving, the original is duplicated for a printing-plate and then stowed away. The electrotyper first cleans the plate with potash,

then coats it with the thinnest possible film of silver to prevent the original from adhering to the alto. It is then placed in a frame in a vertical vat holding a solution of sulphate of copper, and is connected with a battery; copper is slowly deposited upon its face for a day or two. The deposit in a vertical vat is the result of very slow action and presents a smooth and even surface all over the face of the plate.

The plate is next placed in a horizontal vat, and the process of depositing the copper is proceeded with more rapidly. It is daily removed and weighed to ascertain the amount it has received, and the protuberances which form on the back are filed down to make a smooth surface. After a few days, when it has received a sufficiently thick coating, it is removed, and it then appears like a single, thick plate, as the deposit and the original plate are joined at the edges. The edges are filed and the plates separated, the deposit being in alto-relievo. The same operation is repeated, using the alto on which to deposit, and the result is an exact duplicate of the original.

About thirty pounds of copper are deposited for a large-sized alto, and fifty pounds for a basso. When the life of this basso is gone, the alto first obtained is used to produce another basso, and so on for perhaps five or six times, or until the alto is used up, when another strong one must be obtained from the unused original basso.

The question of fine printing is almost, if not quite, as important as that of fine engraving. It is almost impossible to obtain a plate of uniform thickness, and as the rolls of the printing press do not allow for irregularities, except to a small degree, the impress will show strong at the thick, and weak at the thin parts. The printer first lines up the rolls to the average thickness of the plate, and takes an impression. He then cuts out pieces of paper of the shape of the weak parts and puts them under the back of the plate, and runs it through. again. This time it looks better, but he continues cutting out pieces and pressing up more and more upon the back of the plate at the faulty places, until he has a perfect proof. After the day's printing is completed, the charts are dried and then placed in a large press and subjected to a pressure of about two hundred and fifty tons, which takes out all the irregularities and forces the ink into the paper so that it will not readily rub in use.

Dry printing is impossible with an engraved plate, and wetting the paper is open to serious objection because of the shrinkage in drying. The paper supplied from France shrinks the least and also the