Availability of machine divided into 360 degrees gave birth to accurate horizontal circle
of angle measuring instrument theodolite.Development of micro-electronics brought in electronic or digital theodolite where angle readings are displayed on a screen and electronically recorded in instrument's electronic memory.
This device is not otherwise mentioned in the literature, and its originality is doubtful, since as early as 1810 Malus constructed a horizontal circle goniometer with observation telescopes.
No goniometer with a horizontal circle and mirror is known to the author, but Mallard illustrated one in 1879, and professor William Hallowes Miller (1801-1880) of Cambridge, for whom the currently used crystal indices ("Miller indices") are named, published a description of such a goniometer, manufactured by Troughton & Simms, in 1876.
All of these disadvantages can be avoided by the use of a horizontal circle.
Strictly for historical reasons, the introduction of observation telescopes in vertical and horizontal circle goniometers will be described separately: Malus's goniometer first and Mitscherlich's second.
The Malus goniometer has a horizontal circle, and a straightedge, rotatable about the circle axis, on which the crystal is actually mounted.
The horizontal circle H is divided into single degrees, and may be read to half a degree with the aid of an indicator a.
The horizontal circle H corresponds to the equator on a globe, the vertical circle a meridian and its 0 point the north pole.
When the tail of Comet Hyakutake was at its longest in the western sky in the small hours of the night, I saw much of it at nearly the same altitude, tracing a large section of a horizontal circle
. There is one other circumstance in which I've beheld such an arc - in fact an entire horizontal circle
around the sky.
The horizontal circle readings are recorded at the point at which the gyro mark reverses direction.
The observer then enters four horizontal circle readings, two on face II and two on face I, taken to the Reference Object (RO).