visions of the rod lying between the outside hairs in the eyepiece indicates with quite surprising accuracy the distance away the rod is from the instrument. The alidade is also fitted with a vertical arc, by means of which elevations are determined when necessary.

When a plane-table party goes into the field, it is supplied with a blank sheet -the most important and most carefully guarded part of the equipment. This sheet is made of heavy Whatman drawing paper, cut so that it will have the least shrinkage and expansion from dampness and dryness, and is backed with heavy cloth. Sometimes there are two pieces of paper, with grains running in opposite directions, and cloth between, where great extremes of temperature and dampness are expected. On this blank sheet are drawn lightly the meridians and parallels; and in these squares are indicated, by triangles inclosing dots, the positions of the various triangulation stations, and other signals by means of circles and dots, from which the party is to work in drawing the actual topography.

HOW THE PLANE TABLE IS CARRIED FROM STATION TO STATION.

This work of laying off a blank sheet and locating the position of signal points, is done in the office, and on its accuracy depends in great measure the accuracy of the finished chart.

The complete equipment of a planetable party is as follows:

One plane table; two rods; one alidade and equipment; one or more blank sheets; one umbrella; one launch, for river and coast work; one tender; from four to six men; and axes, shovels, tools, glasses, clocks, compasses, etc. If any inland or traverse work is to be done, the equipment will include a team and

wagon.

How the Field Work is Done

Let us suppose that the party is comfortably located at some resort, farm house, or other place of living, as nearly central to the work in hand as possible. The first thing in the morning, after the weather prophets have assured the topographer in charge of the party that no immediate rain is in prospect, is to load all the instruments into the launch. The party then proceeds to the signal from which work is to commence. If the signal is still standing, well and good. In the center of it will be found a large peg driven into the ground; and in the peg, a nail, which indicates the exact location of that particular "point." If the signal has been destroyed by wind or water, or the malicious ignorance of the people of the locality, it must be rebuilt. If the peg is gone, the party digs down three feet or more until a bottle or stone jar

comes to light, also with a mark upon it, indicating again the position of that "point." This mark, called a ground mark, is of course not found beneath church steeples or tall chimneys, or temporary signals, but is a most important part of the erection of a triangulation signal, inasmuch as if it were omitted, and the surface mark obliterated, recovery of the point would mean much triangulation work for a party sent out to do work of another kind.

We shall suppose that the signal has been recovered and put in order. The plane-table man, who carries and has charge of that instrument, now removes the protecting top of the movement, and erects the tripod over the ground peg, or as close to it as he can get. The table itself has previously been removed from its case, and attached by the proper screws to the movement. The sheet is then taken from its waterproof copper cylinder, and stretched across the top of the table, so that the signals to be used are on the plane surface. The rest of the chart is rolled up and secured under the edges of the table with metal clips. The alidade is now placed upon the table, and the table leveled by means of its leveling screws, with the aid of a level located upon the base of the alidade. Meanwhile, some other member of the party has been engaged with glasses in locating other signal points about the coast-line -or across the river, if river work is being done. Three are essential, and more

are preferred, although two may be used in an emergency.

Let us suppose that the various signals are known by number, as 1, 2, 3, etc. Actually they have all sorts of namessuch as "Colton's Point," "Blue Sow No. 2," "Higgins Creek," etc.-names taken from the nomenclature of the country. The party has located over No. 1. The sheet is on the table, so adjusted (for convenience only) that the little dot and triangle representing signal No. I is somewhere near the center. The alidade is now placed upon the chart so that one of its straight edges just touches No. I and, further along, No. 2. The table is now revolved until signal No. 2 can be seen on the central cross-hair in the telescope. The table is then clamped. The alidade is now lifted and placed so that its edge touches No. I and No. 3. If, without movement, No. 3 signal is exactly on the cross-hair of the telescope, the table is considered "oriented;" that is, the sheet upon the table, with its dots representing signals, bears an accurate and exact relation to the same signals

The table having been accurately oriented, actual charting is to commence. The two men who act as rodmen unfold their rods, and erect them, fastening them by means of screws provided for the purpose. The rodmen separate, going in opposite directions along the coastline. They stop when they reach the first bend, indentation, or promontory of any size, and turn, facing the instrument under the signal, holding their rods upright, with the scale as nearly square to the instrument as possible. The topographer now orients the alidade so that the image of one of the rods appears across the cross-hairs, being particular to see that one edge of the base of the instrument passes over the point on the chart which. is the location of signal No. 1, over which his instrument is erected. He reads in the telescope the number of meters distant the rodman is from him- thirtythree, let us say. He then takes a pair of dividers, and measures off with their points, 33 meters from a metal scale which is reduced twenty thousand times; in other words, the chart is being made on a scale of 1 to 20,000. He now has

at hand the following data: - the distance away the rodman is, actually; the distance away his rod should be on the chart in the reduced ratio; and, indicated by the edge of the alidade, the direction from base in which the rodman is. With a fine-pointed pencil, therefore, he draws along the edge of the alidade a light line, the length of the distance between his compass points. He then, by a certain prearranged signal- generally a movement of the arms over the head-signifies to the rodman to move on; and turns his attention and his instrument to the other rodman, with whom he repeats the process just described. When he gets again around to the first rodman, and takes the distance, measures it on the compass, etc., he does not draw a line along the alidade ruler, but makes a point on the sheet, with his pencil, on the edge of the ruler, and the compass distance. This point is then connected to the end of the first line drawn.

It is obvious that this process, kept up, will produce a reduced facsimile of the coast-line which is accurate as to distance, direction, and contour. This is the elementary essential of plane-table work. There comes a time when the rods can no longer be read-when they appear so small in the telescope as to preclude accuracy, or when a bend in the coastline, or undergrowth, prevents their be

around and sighting on the signal, with the alidade edge running over the rod station as plotted, and the signal point on the chart, the table is again oriented, and checked up, if necessary, by observations on other signal points. From this station the work goes on as before; only in this case both rodmen are forward rod

charts. The surroundings of the coast are of great importance to navigators. They want to know the location of roads, the location of villages, and the character of the shore. Accordingly, much work is done a short distance inland. Here it sometimes becomes necessary to run what is called a traverse line, in which