From the Sung Shih ("History of the Sung"),

compiled in 1345 from sources many years older:

In the 5th year of the Thien-Shêng reign-period of the emperor Jen Tsung (+1027), Yen Su, a Divisional Director in the Ministry of Works, made a south-pointing carriage. He memorialized the throne, saying, [after the usual historical introduction]:

Throughout the Five Dynasties and until the reigning dynasty there has been, so far as I know, no one who has been able to construct such a vehicle. But now I have invented a design myself and have succeeded in completing it.
The method involves using a carriage with a single pole (for two horses). Above the outside framework of the body of the carriage let there be a cover in two storeys. Set a wooden image of a hsien (immortal) at the top, stretching out its arm to indicate the south. Use 9 wheels, great and small, with a total of 120 teeth, i. e.

(A) 2 foot-wheels (i.e. road-wheels, on which the carriage runs) 6 ft. high and 18 ft. in circumference,

(B) attached to the foot-wheels, 2 vertical subordinate wheels, 2.4 ft. in diameter and 7.2 ft. in circumference, each with 24 teeth, the teeth being at intervals of 3 inches apart,

(E) then below the crossbar at the end of the pole two small vertical wheels 3 inches in diameter and pierced by an iron axle (iron axles according to another translator),

(D) to the left 1 small horizontal wheel, 1.2 ft. in diameter, with 12 teeth, and to the right 1 small horizontal wheel, 1.2 ft. in diameter, with 12 teeth,

(C) in the middle 1 large horizontal wheel, of diameter 4.8 ft. and circumference 14.4 ft., with 48 teeth, the teeth at intervals of 3 inches apart,

(V) in the middle a vertical shaft piercing the centre (of the large horizontal wheel) 8 ft. high and 3 inches in diameter; at the top carrying the wooden figure of the hsien.

When the carriage moves (southward) let the wooden figure point south. When it turns (and goes) eastwards, the (back end of the) pole is pushed to the right; the subordinate wheel attached to the right road-wheel will turn forward 12 teeth, drawing with it the right small horizontal wheel one revolution (and so) pushing the central large horizontal wheel to revolve a quarter turn to the left. When it has turned round 12 teeth, the carriage moves eastwards, and the wooden figure stands crosswise and points south. If (instead) it turns (and goes) westwards, the (backs end of the) pole is pushed to the left; the subordinate wheel attached to the left road-wheel will turn forward with the road-wheel 12 teeth, drawing with it the left small horizontal wheel one revolution, and pushing the central large horizontal wheel to revolve a quarter turn to the right. When it has turned round 12 teeth, the carriage moves due west, but the wooden figure stands crosswise and points south. If one wishes to travel northwards, the turning round, whether by east or west, is done in the same way.

It was ordered that the method should be handed down to the (appropriate) officials so that the machine might be made.

In the first year of the Ta-Kuan reign-period (+1107), the Chamberlain Wu Tê-Jen presented specifications of the south-pointing carriage and the carriage with the li-recording drum (hodometer). The two vehicles were made, and were first used that year at the great ceremony of the ancestral sacrifice.

The body of the south-pointing carriage was 11.15 ft. (long), 9.5 ft. wide, and 10.9 ft. deep.
(A) The carriage wheels were 5.7 ft. in diameter, the carriage pole 10.5 ft. long, and the carriage body in two storeys, upper and lower. In the middle was placed a partition. Above there stood a figure of a hsien holding a rod, on the left and right were tortoises and cranes, one each on either side, and four figures of boys each holding a tassel.
In the upper storey there were at the four corners trip-mechanism, and also 13 horizontal wheels, each 1.85 ft. in diameter, 5.55 ft. in circumference, with 32 teeth at intervals of 1.8 inches apart. A central shaft, mounted on the partition, pierced downwards.
(C) In the lower story were 13 wheels. In the middle was the largest horizontal wheel, 3.8 ft. in diameter, 11.4 ft in circumference, and having 100 teeth at intervals of 1.25 inches apart.
(D) (On vertical axles) reaching to the top (of the compartment) left and right, were two small horizontal wheels which could rise and fall, having an iron weight (attached to) each. Each of these was 1.1 ft. in diameter and 3.3 ft. in circumference, with 17 teeth, at intervals of 1.9 inches apart.
(B) Again, to left and right, were attached wheels, one on each side, in diameter 1.55 ft., in circumference 4.65 ft., and having 24 teeth, at intervals of 2.1 inches.
(F,G) Left and right, too, were double gear-wheels (lit. tier-wheels), a pair on either side. Each of the lower component gears was 2.1 ft. in diameter and 6.3 ft. in circumference, with 32 teeth, at intervals of 2.1 inches apart. Each of the upper component gears was 1.2 ft. in diameter and 3.6 ft. in circumference, with 32 teeth, at intervals of 1.1 inches apart.
(H) On (Above according to another translator) each of the road-wheels of the carriage, left and right, was a vertical wheel 2.2 ft. in diameter, 6.6 ft. in circumference, with 32 teeth at intervals of 2.25 inches apart.
(E) Both to left and right at the back end of the pole there were (The left and right back poles each had according to another translator) small wheels without teeth (pulleys), from which hung bamboo cords, and both were tied above the left and right (ends of the) axle (of the carriage) respectively.

If the carriage turns to the right, it causes the small pulley to the left of the back end of the pole to let down the left-hand (small horizontal) wheel. If it turns to the left, it causes the small pulley to the right of the back end of the pole to let down the right (small horizontal) wheel. However the carriage moves the hsien and the boys stand crosswise and point south.
The carriage is harnessed with two red horses, bearing frontlets of bronze...

Source: Needham. Joseph: Science and Civilization in China, Volume 4, Part II, 1965