"At sea the declination is generally observed by means of an azimuth compass invented by Kater. It consists of a magnet with a graduated compass card attached to it. At the side of the instrument opposite the eye there is a frame which projects upwards from the plane of the instrument in a nearly vertical direction, and this frame contains a wide rectangular slit cut into two parts by a wire extending lengthwise. The eye-piece is opposite this frame, and the observer is supposed to point the instrument in such a manner that the wire above mentioned shall bisect the sun's visible disk. There is a totally reflecting glass prism which throws into the eye-piece an image of the scale of the graduated card, so that the observer, having first bisected the sun's disk by the wire, must next read the division of the scale which is in the middle of the field of view." —The Encyclopedia Britannica, 1903

Azimuth Compass

"At sea the declination is generally observed by means of an azimuth compass invented by Kater. It consists…

"When a line of direction falls within the base, a body stands when not, it falls... On the same principle, a load of stone may pass safely over a hillside, on which a load of hay would be overturned [as shown by the line of direction in this illustration]." —Quackenbos 1859

Line of Direction from the Center of Gravity of an Object

"When a line of direction falls within the base, a body stands when not, it falls... On the same principle,…

A compass that tells direction.

Compass

A compass that tells direction.

An early compass.

Compass

An early compass.

A tool used to tell direction.

Compass

A tool used to tell direction.

A compass placed in some convenient part of a ship on the midship line and provided with vanes, screws, and other apparatus for observing the bearings of heavenly and terrestrial objects.

Azimuth Compass

A compass placed in some convenient part of a ship on the midship line and provided with vanes, screws,…

"It has four cardinal points, N, E, S, and W marked, as well as a circle graduated in degrees from sero to 360 degrees, clockwise around the circle." — Moss, 1914

Box compass

"It has four cardinal points, N, E, S, and W marked, as well as a circle graduated in degrees from sero…

Illustration of a man using a cross-staff, a navigation tool used largely at sea. It was originally created for use in land navigation and finds latitude by measuring the distance of a star above the horizon. It cannot find longitude. The man in the drawing has the cross-staff positioned with the rod on his cheek. There is a star above him. He is wearing a wide, floppy-brimmed hat.

A Cross-Staff Used for Navigation at Sea

Illustration of a man using a cross-staff, a navigation tool used largely at sea. It was originally…

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side of an island, obstruction, or gore in the roadway. Traffic separated by this sign may either rejoin or change directions.

Double Arrow, Black and White

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side…

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side of an island, obstruction, or gore in the roadway. Traffic separated by this sign may either rejoin or change directions.

Double Arrow, Color

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side…

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side of an island, obstruction, or gore in the roadway. Traffic separated by this sign may either rejoin or change directions.

Double Arrow, Outline

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side…

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side of an island, obstruction, or gore in the roadway. Traffic separated by this sign may either rejoin or change directions.

Double Arrow, Silhouette

The Double Arrow sign may be used to advise road users that traffic is permitted to pass on either side…

An illustration of military personnel in formation changing direction.

Military Personnel in Formation Changing Direction

An illustration of military personnel in formation changing direction.

An illustration of military personnel in formation picking up full pace after changing direction.

Military Personnel Formation Marching

An illustration of military personnel in formation picking up full pace after changing direction.

"Apparatus used to detect current and its direction." -Avery 1895

Galvanoscope

"Apparatus used to detect current and its direction." -Avery 1895

An illustration of a man deciding between two paths.

Man Deciding Between Two Paths

An illustration of a man deciding between two paths.

"Illustrating Maxwell's 'corkscrew rule' for relative directions of current and lines of force...a current flow through the wire ab in the direction from a to b, the magnetic lines will encircle the wire in the direction of the curved arrow ro which shows the direction in which the corkscrew must be turned to advance in the direction of the arrow n." -Hawkins, 1917

Maxwell's Corkscrew Rule

"Illustrating Maxwell's 'corkscrew rule' for relative directions of current and lines of force...a current…

The aggregate of the components of momentum.

Components of Momentum

The aggregate of the components of momentum.

"Fleming's rule for direction of induced current. Extend the thumb, forefinger and middle finger of the right hand [as shown]. Place the hand [so that] the thumb will point in the direction in which the conductor moves, the forefinger in the direction of the lines of force (N to S), then will the middle finger point in the direction in which the induced current flows." -Hawkins, 1917

Right Hand Rule of Induced Current

"Fleming's rule for direction of induced current. Extend the thumb, forefinger and middle finger of…

"A slender length of brass resting in a small brass button and designed to stand vertically on the center of a compass glass. From the shadow cast by it the sun's bearing is determined. It may also be used to take compass bearings of other objects." -Whitney, 1911

Shadow Pin

"A slender length of brass resting in a small brass button and designed to stand vertically on the center…

These are skew bevel wheels, made of a hyperboloid of revolution around an axis.

Sliding Contact

These are skew bevel wheels, made of a hyperboloid of revolution around an axis.

"Direction of the needle when placed at a right angle to the uniting wire." -Comstock 1850

Wire at Right Angle to the Uniting Wire

"Direction of the needle when placed at a right angle to the uniting wire." -Comstock 1850

"Direction of the needle when placed vertical next to the uniting wire." -Comstock 1850

Wire at the Vertical of the Uniting Wire

"Direction of the needle when placed vertical next to the uniting wire." -Comstock 1850

"To change the direction, it is only necessary that the rope by which the weight is to be raised, should be carried in a line perpendicular to the axis of the machine, sto the place below where the weight lies, and there be let fall over a pulley." -Comstock 1850

Wheel and Axle

"To change the direction, it is only necessary that the rope by which the weight is to be raised, should…