Parallel rulers on the principle of Field's are strongly recommended for chart work, as they combine in a single instrument the advantages of a parallel ruler and a 180-degree protractor. Any direction can be read or laid off by simply moving the parallel ruler to the nearest projection line, which is a process not only more convenient than referring to the compass rose printed on the chart but also more accurate because of the longer radius. These instruments can also be used the same as a plain parallel ruler. Field's parallel rulers are made in two forms, one rolling and the other sliding. The former is a single ruler with edge graduated 90 degrees either way, and mounted on rollers; it is the most rapid instrument for reading or laying off a direction, but it requires a smooth surface. The latter is an ordinary two-bar parallel ruler with edge when closed graduated 90 degrees either way; it is a very serviceable instrument and probably more to be depended upon for ordinary use than the rolling form. Some form of combined protractor and parallel ruler should be in every navigational equipment, and it is unfortunate that these instruments are not better known in this country. There are other forms of half-circle protractors which are used on the same principle, that is, of bringing the center on to a projection line and reading where the line cuts the border graduation of the protractor. Thus a semicircular protractor is used with a separate straight edge, along which it is slid to the nearest meridian; another form is the simple circular protractor with a thread fastened at the center. All these forms of protractors, it will be noted, are intended to work from the true meridian, and they are usually graduated in degrees only; the use of degrees instead of points is becoming much more general in navigational work, and reference to the true meridian is also more common than formerly. The standard three-arm protractor, or station pointer, as it is known to the English, should be a part of every navigational outfit because of its value in locating a position by the three-point problem. A recent American invention, Court's three-arm protractor, is an instrument made of celluloid for the same purpose. It should not be considered as a substitute for the standard metal instrument, but it is a simple, cheap, and handy supplement to it, as it may be readily used for small angles and short distances where there are mechanical difficulties in working with the metal three-arm protractor. Other protractors can be used for the three-point problem, as, for instance, Cust's protractor on celluloid, on which the angles are drawn in pencil and erased, and the tracing-paper protractor.