The Mirror Compass

A Brunton-type magnetic compass for less than $10!


A discount tool merchant (Harbor Freight Salvage) recently offered a brass compass for less than $10, implying that it would make an interesting desk ornament. A magnetic compass for this price must certainly be a bargain one way or another, so I purchased one. It turned out to be a Brunton-type compass made in India, weighing 500 g, certainly an impressive piece of brass, and reflecting a high level of metalworking skill. The availability of this compass for such an extremely low price, a twentieth of the normal price, is remarkable. A photo of the compass is shown at the right. Since Brunton is really a trade name, I'll call compasses of this type "mirror compasses" instead. In fact, this compass is more than an ornament, and can be used for the practical measurement of angles if it is handled properly. No instructions came with the instrument, so I hope to supply the lack here. Possibly one reason for its low price is that few know how to use it any more.

Small compasses are usually called pocket compasses, which are easily carried and used without other accessories. The two principal kinds of pocket compasses are the lensatic or prismatic compass, the typical military compass, used in map reading, and the mirror compass, of which an example is the Brunton "pocket transit" compass, made by Ainsworth. The Brunton is a typical geologist's compass, and is well adapted for geological field work, which seems to be rare these days. It folds up into a compact form for carrying. The lensatic compass is discussed in The Lensatic Compass, while this article treats the mirror compass. Compasses must be made of nonmagnetic materials, such as brass, aluminium or plastic, so it is clear why this compass is made of brass.

All magnetic compasses use the earth's magnetic field to supply a reference direction for the measurement of horizontal angles. This direction has a fundamental significance only when the directions on the earth's surface are to be specified, and then the magnetic declination must be taken into account. These matters are treated in the article on the lensatic compass, and will be ignored here. The direction of a line can be specified by its azimuth, the angle the north end of its horizontal projection makes with north (magnetic or true), measured clockwise from 0° to 360°. The "north" end of the line is simply an expression of its sense. Any line has azimuths differing by 180°, depending on the sense chosen.

The reference direction for vertical angles is the direction of gravity, as determined by a plummet or a bubble level. The slope of a line is the angle it makes with the horizontal, measured from 0° to ±90°. Slope can also be expressed in percent gradient, equal to the ratio of vertical to horizontal displacement, times 100%. A 45° slope is a 100% gradient.

The magnetic needle may act as an index to a divided circle fixed to the compass body, as in the compass under discussion, or may be mounted on a graduated disc free to rotate with an index fixed to the compass body, as in the lensatic compass. The divided circle of this compass is graduated from 0° to 360° anticlockwise, and has been rather well done. The needle, unfortunately, has rather blunt tips. It could be improved by attaching the ends of needles or pins with cyanoacrylate cement to the ends of the existing needle, taking care not to disturb the balance. I have not done this, however. The north-seeking end of the needle is identified by red paint. When using the compass, read only one end of the needle in any application, since there is no guarantee that the line between the two ends passes exactly through the pivot. Note that a small plunger on the rim of the compass body controls the needle damper, and holds the needle off the pivot whent he compass is closed.

The compass opens up into two leaves, the lower one containing the magnetic needle, the graduated circle, and a sight, the upper one containing a mirror with a sighting line and a sight. The sighting line is quite fine on this instrument. It is easiest to use the compass when it rests on a firm, level surface. A circular level shows when the compass is horizontal. This is not as important as ensuring that the needle is free to rotate. The front sight should be extended, the slotted part horizontal and the sight itself vertical.

The fundamental problem in handheld compasses is to permit simultaneous sighting and reading of the needle. With a transit, these two actions are separated: first the object is sighted, then the circles are read. This cannot be done with a handheld instrument, since there is no way to hold the instrument in the sighting postion. With a lensatic compass, a lens in the rear sight is used so the close-up scale can be read. With a mirror compass, the reflected view gives the sighting condition, the direct view the reading position. An alternative way of using the compass is to take a sight by any method, then to press down the needle clamp so the needle is held fixed until it can be read. In fact, this method is normally used with the simplest pocket compasses, which have neither lens nor mirror. This method can, of course, be subject to error if the needle moves when it is clamped.

To measure a horizontal angle, look down on the compass from a convenient distance, and adjust the top with its mirror so that you see the front sight in the mirror. Move your eye from side to side, and rotate the compass in a horizontal plane so that the sighting line aligns with the tip of the front sight on the desired object. The compass is now aligned, and the position of the needle read by direct vision. An accuracy of 1° or better can be achieved. Now sight on a second object the same way, bringing object, front sight and sighting line into coincidence. The difference in the azimuths obtained by the two sights is the angle between the lines from the observer to the two objects. Azimuths can be plotted on a map using a protractor. Unlike some compasses, this compass is not a combined compass and plotting instrument.

If the compass is resting on a firm support, another method of sighting is possible. Open the top fully, so that the two sights can be used simultaneously. To use the sights, look through the small aperture near you at the tip of the far sight. When this sight tip aligns with the desired object, the instrument is aligned. The aperture defines the location of your eye to better than 1 mm, and since the distance between the sights is more than 200 mm, the accuracy of setting will be better than a quarter of a degree, which is sufficient. After sighting, you can move your eye to read the circle, taking care that the compass does not budge. This method is usually not as convenient as the first method, the normal one with mirror compasses.

In the lower half of the compass, a small frame carrying an index and vernier will be seen that can be rotated by an arm on the outside of the compass case. This frame carries a bubble level, as well as the circular level for horizontal levelling. Two scales are provided, the inner one in degrees, to ±90°, and the outer one in percent grade, from 0% to ±100%. To measure a vertical angle, you sight from the previous front sight through the small hole in the mirror, with the mirror adjusted so that you can see the frame and its level bubble. The level bubble is round, so it is easily visible, and is very sensitive. Move the lever so that the bubble is centred when the desired object is sighted, and read the scale. If you have a known level, the index error can be determined.

The inclination of a line can also be measured, or at least estimated, by opening the compass fully and lining up the edge of the case with the line whose inclination is wanted. The lever is moved as before until the level bubble is centred, and the index read.

The most frequent geological application is the determination of the strike and dip of a bedding plane. In this case, sighting is not necessary, and the compass is simply read directly when it is held in the proper orientation. To find the strike, hold the compass horizontal, as shown by the circular level, and in contact with the bedding plane. Then read the azimuth of the strike on the graduated circle, and express it as desired. Now hold the compass at right angles to the direction of the strike. This can be estimated by eye well enough in most cases. Move the lever until the level bubble is centred, and read the dip. It is presumed that the edge of the case is parallel to the N-S axis of the compass. If a test shows that it is not, correcting for this error is simple. Note that the prismatic compass is not adapted to measuring strike and dip, since it lacks the reference edge.

The inexpensive Ainsworth Brunton Cadet compass is used in exactly the same way, except that there is no circular level, and the horizontal attitude must be estimated. This compass uses a simple plummet to determine vertical angles.

References

The Brunton pocket transit was invented by D. W. Brunton in 1894. A large variety of instruments will be found at Brunton. A Cadet costs $60 and a conventional Brunton compass costs $265 these days.

C. B. Breed and G. L. Hosmer, The Principles and Practice of Surveying: Vol. I, Elementary Surveying, 11th ed. (New York: John Wiley & Sons, 1977). See pp. 20-29 for use of the compass, and pp. 148-150 for compass surveys for area. The lensatic ("prismatic") compass is only mentioned briefly.

The Wm. Ainsworth, Inc. company of Denver, which was founded in 1880, is no longer extant. Its disappearance is a loss.


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Composed by J. B. Calvert
Created 8 November 2003
Last revised