The advent of the gyro-compass with its pelorus repeaters, made necessary the design of an instrument for use with the repeaters, that would fulfill all conditions to be met. The U. S. Naval Observatory courteously authorized the writer to make alterations to two Type III azimuth circles and the Type V is the result. The writer worked with the two altered circles under all conditions of weather for several months and found no condition not met.
The circle was designed, primarily, for use in connection with the gyro-compass, but may be efficiently used with the magnetic compass; however, it is considered better practice to take all observations with the gyro repeater and obtain magnetic compass errors by comparative readings of the ship's head. When used on a gyro-compass repeater, at night, no illumination is required other than that supplied by the gyro repeater itself, but when used on a magnetic compass at night, some arrangement must be made for illuminating the compass card, unless the Mark VII binnacle is installed which has the compass card illuminated.
The repeater on which the instrument is used should have a double row of degree figures, one row erect and the other row inverted. The inverted row is for use when using the mirror for reflecting the degree graduations to the eye, and the other row for direct readings. The two rows of figures may cause slight confusion at first, but that will wear off in a very few days.
The features sought and accomplished in the design of the instrument are as follows:
(a) Ease of setting to any desired angle from the ship's head.
(b) Ease of use in the daytime on near-by objects.
(c) Ease of use in the daytime on distant objects.
(d) Ease of use in the night-time.
(e) Ease of use in rainy weather.
(f) Ease of use in the daytime for stir; bearings.
(g) Ease of use in the night-time for star bearings.
(h) Ease in adjusting on board ship.
The details of the Type V circle are shown in the illustrations, but a brief description will be given of how the instrument is constructed and how each object is attained.
The circle is manufactured with the horizontal wire in the cross-plate accurately centered and so that it can be renewed without disturbing the adjustment. The adjustment of all elements are referred to this wire.
(a) Ease of setting the instrument to any desired angle from the ship's head is accomplished by cutting away the circle on the front side in such a way the graduated bezel ring can be seen and angle figures on it easily read by looking between the front plane mirror and the front vane. (This is not possible as the circle is now being manufactured.)
(b) The ease of use in the daytime on near-by objects is accomplished by fitting the front vane with a point and the rear vane with a V for sighting over. The vertical wire in the front vane and the slit in the rear vane are fitted but will probably seldom be used. The front plane mirror is fitted partly to reflect the image of the repeater card to the eye. The telescope is fitted primarily for use on distant objects, the bearing being read direct by the horizontal wire, but experience has proved its value and desirability for near-by work, except in wet weather.
(c) Ease of use in the daytime on distant objects is accomplished by means of the telescope, which was designed to be quickly attached to or detached from the circle and if used with care two dowel pins on the mount setting into the cross-plate will insure against adjustment error. (As the circle is now being manufactured, the thumb screw for securing the telescope to the cross-plate is cumbersome and easily lost and should be replaced by a simple, quicker and non-losable arrangement.)
(d) Ease of use in the night-time is achieved by mounting a point of radium salts on the front and the rear vanes and as the radium salts are luminous at night these points make two or the three points in the line of sight. The repeater should be dimmed to prevent the blinding of the eye by the light reflected to it by the front plane mirror. (On the circle now being issued the front point of radium salts is too small.)
(e) For use in rainy weather, the front plane mirror is fitted in lieu of a prism, for, by throwing up the mirror to the vertical position and smearing the moisture with the thumb, and throwing down to position, it can be efficiently used without eye-strain and there will be no distortion.
(f) In taking bearings of the sun, the curved mirror and prism used on the Type III circle are fitted at 45° from the cross-plate for the use of those who desire to use that method. For those who prefer the other method, a curved mirror is mounted on the rear of the cross-plate of the instrument and a white line is marked on the center of upper part of the front mirror frame on which to split the pencil of light. For leveling purposes a level is mounted on the under side of the cross-plate.
(g) To take star bearings, a plane mirror is mounted on the rear of the cross-plate of the instrument for reflecting the image of the star to the front mirror, which reflects the image back to the eye. The image of the star is centered in the front mirror by means of a point of radium salt in the center of the upper part of the front mirror frame. The instrument is leveled by using the level under the cross-plate which has the level case so cut away that the bubble can easily be seen by means of the light coming up through the translucent repeater card.
(h) The instrument is capable of complete adjustment in the chart-house except for that part used for taking sun bearings, which was taken from the Type III circle. The adjustment feature is a distinct advantage; for, with the Type III circle, a navigator never knew for sure how much error was in his circle.
As was stated before, the circle is manufactured with the horizontal cross-wire accurately centered and the adjustment of all parts is referred to as follows:
Rear Vane.—Throw down the vane and adjust it so that the V is exactly vertical to the horizontal wire in the cross-plate and then place the eye vertically over the slit and V. Swing the vane slowly up and if the relative position of the horizontal wire in the cross-plate and the V and slit in the vane remain unchanged, the vane is adjusted correctly.
Front Vane.—The front vane may now be adjusted in a similar manner to the rear vane.
Front Plane Mirror.—Set the mirror bracket so that when the mirror is thrown up to a vertical position the white lines on the mirror frame coincide with the horizontal wire in the cross-plate. Place the eye at the rear vane and throw the mirror so as to reflect the horizontal wire in the cross-plate to the eye and give it the maximum swing obtainable back and forth by which it will reflect to the eye the horizontal wire or the rear vane. If the reflected images remain vertical and coincident with the lines on the mirror frame, the mirror is in adjustment.
Rear Mirrors.—Ad just in a similar manner to the front mirror.
Telescope Attachment.—Sight the vanes of the circle on some object on the ship as the flag staff and if the vertical wire of the telescope coincides with this object, the telescope is in adjustment.
The curved glass mirror and prism that are mounted 45° from the horizontal wire in the cross-plate, may be adjusted by making comparative observations of the sun with it and the element mounted on the cross-plate.
The uses of an azimuth on board ship are limited to the following objects:
(A) Setting to any angle relative to the head of the ship.
(B) Taking compass bearings of terrestrial objects.
(C) Taking compass bearings of celestial bodies.
These objects are all quickly and readily accomplished in the following manner:
A (1) To set the circle to a desired angle from the ship's head.
(a) Move front plane glass mirror so that it makes an angle of about 45° with the horizontal wire in the cross-plate.
(b) Swing the circle around and set it to the desired angle by reading the graduation on the graduated bezel ring by means of the horizontal wire in the cross-plate.
B (1) To take a bearing, in the daytime, of a terrestrial object, using the sighting vanes.
(a) Throw down the rear curved and plane mirror.
(b) Place the eye at the rear vane and swing up the front plane mirror until it reflects to the eye the outer row of figures and degree graduations on the compass card. (This reflects the inverted figures and shows them erect and obscures the erect figures which the reflection shows inverted and thus is avoided the initial confusion resulting from seeing the two rows of figures. This confusion will disappear after working for a short time with the double-row card.
(c) Swing the circle around to the approximate bearing of the object to be observed.
(d) Place the eye at the rear vane and bring the V of the rear vane, the point of the front vane and the observed object in line.
(e) Drop the eye (without moving the head) and read the bearing as reflected by the front plane mirror.
B (2) To take a bearing, in the night-time, of a terrestrial object, using the sighting vanes.
Bearings at night will be taken in a manner similar to that employed in the daytime except that the points of light on the vanes will be used instead of the point and V. The reflector, however, should be dimmed so as not to blind the eye.
B (3) To take a bearing of a distant object with the telescope attachment.
(a) Throw down the rear vane.
(b) Swing the circle around to the general direction of object to be observed (a little practice will make it possible to always bring the object within the field of the telescope without delay or inconvenience).
(c) Bring the observed object and the vertical wire in the telescope into coincidence. The horizontal wire in the telescope should be approximately horizontal to avoid a possible small amount of parallax.
(d) Drop the eye quickly (it should not be necessary to move the head) and read off the bearing direct by means of the horizontal wire in the cross-plate. (If difficulty is experienced in seeing the compass card without moving the head, the eye lense end of the telescope may be depressed a small amount to overcome this fault without introducing an error.)
C (1) To take a bearing of the sun.
(a) Swing the circle around to the approximate bearing of the sun.
(b) Place the eye at the V in the rear vane.
(c) Throw down the rear plane mirror.
(d) Throw up the front plane mirror until it shows the reflected image of the graduations and the outer row of figures on the compass card.
(e) Swing the curved mirror to bring the pencil of light on the upper edge of the front mirror frame.
(f) Level the instrument by looking at the level on the under side of the cross-plate.
(g) Swing the circle to the right or left as necessary to split the pencil of light on the vertical white line on the upper side of the front mirror frame.
(h) Read off the bearing by the front plane mirror.
C (2) To take a bearing of a star.
(a) Throw the curved mirror well back out of the way and throw the rear plane mirror down.
(b) Dim the repeater.
(c) Swing the circle around to the approximate bearing of the star to be observed.
(d) Place the eye at the V of the rear vane.
(e) Throw up the front plane mirror until the plane of the mirror is perpendicular to the line of sight.
(f) Swing the rear plane mirror slowly up until the image of the star is seen in the front plane mirror and near the upper edge of it.
(g) Drop the eye to the level on the under side of the cross-plate and level the instrument.
(h) Swing the circle to the right or left as necessary to bring the star image directly under the point of luminous radium salts located on the upper side of the front mirror frame.
(i) Drop the eye to the compass card and read the bearing of the star directly from the card by means of the horizontal wire in the cross-plate.
In connection with the above description of the Type V azimuth circle, attention is particularly invited to that part of it which is designed for use in taking star bearings. Many navigators do not feel the need of star bearings for compass error, but the ability to get them has on several occasions contributed considerable to the writer's peace of mind; once, when making the unlighted entrance to a narrow channel before sun up and on other occasions when it was suspected that the gyro-compass was oscillating. The compass error by the star bearing attachment is considerably simplified when azimuths can be obtained of Polaris and thence the compass error without the use of the time or tables.
It might be said in closing, that, on the Montana, the writer obtained and installed water-tight seven point plugs and receptacles in the electric leads from the connection boxes to the repeaters for the purpose that, should the gyro repeater fail on the gyro-compass itself, the repeaters could be quickly detached, removed and the peloruses installed in the stands for use, instead of trying to work a repeater as a dumb compass.
Note by the Writer.—The Type V azimuth circle now being issued to the service is not correct in manufacture, but it is hoped that corrections will be made. The defects are—
(a) The front plane mirror is too close to the front vane, thus precluding the seeing of the graduated bezel ring and the degree figures when looking between the mirror and the front vane.
(b) The center of weight of the telescope and mount is not perpendicular to the center of the instrument, thus necessitating the use of counter weights.
(c) The thumb screw for securing the telescope to the instrument is cumbersome and easily lost. The attachment arrangement should be simple and parts non-losable.
(d) The radium spot on the front vane is too small.