This section of Mercury and his Children (c1475-85) needed a post by itself, because I found doing the research on the history of clocks and the quadrant to be fascinating and somewhat distracting.
By 1500, few towns were without some tower clock, but domestic clocks, though widely diffused among the wealthy, were not common in Europe as a whole until a later period. Later writers imply that clockmaking was so highly developed in Nuremberg in the fifteenth century that domestic clocks came into more general use in central and southern Germany than elsewhere in Europe. These German clocks of the fifteenth century were among the first made to indicate minutes and seconds, and some use was made of them by astronomers. Waltherus, Landgrave of Hesse, a pupil of Regiomontanus, made use of a clock in astronomical observations as early as 1484, commonly regarded as the earliest application of the mechanical clock to scientific use. We are told that the clock measured accurately the interval between the transits of the sun from noon to noon.If this is mostly correct, then the dating of Mercury and his Children to somewhere between 1475-85 is pretty close to spot on, since it appears as if the gentleman in this section of the drawing is doing just as Mr. Usher describes at the end of the above quote.
From what little information I can find, the inner workings of the above clock are likely spring related as opposed to weight driven. Weight driven clocks tended to be much larger and a lot less portable. Spring driven clocks were supposedly invented sometime in the early 15th century. Though it may look otherwise, there is most likely only one hand on the clock face, as seen in the following picture of a portable clock of the mid-16th century found at the Metropolitan Museum of Art.
Unfortunately, this is where my foray into clock history ended, because very few other sources gave any information which would actually enlighten this subject further.
On the other hand, I found the astrolabe quadrant used in this drawing and some information on it.
The instrument in the picture is a rare example of especially ingenious medieval European instrument-making (ca. 1325 AD). The sighting arm rotates across a pattern made by folding the circular face of an astrolabe into quarters. The user can not only take star sights as with a quadrant, but make calculations as with a simple astrolabe. Essentially, the circular face of an astrolabe has been "folded over" twice to create a quarter-circle. This instrument could serve as a measuring tool and perform many of an astrolabe's calculation functions as well.And the reverse...
A little more info on the astrolabe quadrant can be found at The Astrolabe Quadrant.