De Magnetica [...] Plantarum p. 644

sets they all close up, and when it rises they open, and in them this motion is seen to have such great force that when they have pods they close them up most securely with their leaves and seem to hold them in, but when the sun rises they open and release the pods. I have noticed that this turning of the leaves is shared by many other Egyptian plants, such as Acacia, Abrus, Absus, Sesban and Tamarinth. That is what he says. In the same way Athenaeus asserts that Hemerocallis (Day Lily) withers as the sun sets and sprouts anew when it rises again. Thus all the plants I have mentioned can serve to indicate the hours even when the sky is cloudy, albeit imperfectly, as they somewhat impede the circular motion of the flower by the resistence of the stalk. So by removing all obstructions and balancing the flower magnetically I have discovered that its operation demonstrates the time of day accurately enough. But since this kind of botanical timepiece is worth seeing, I thought its construction should be demonstrated here to the curious reader.

PROBLEM.

To construct a Botanical Timepiece or Clock using Heliotropic plants.

Since many people have derived great pleasure from clocks of this kind made by me elsewhere, they have urgently requested me to put the method for constructing them into the public domain, which I thought I could most appropriately do now; so this is what you do: Take a large and capacious tub and partly fill it with water; in the middle of its base fix a spike, to which you will fit cork, according to the size of the pot and stem of the Heliotrope, in such a way that the cork can move easily and without obstruction around the spike as if round an axle: bind to this cork a Heliotropic flower which you know to be imbued with particularly efficacious quality, together with its root and stem, in such a way that it rests upright on the cork, or even passes through it into the water, as shown in the accompanying diagram: tie this stem around the roots with bundles of wool which hang down into the water, for the water drawn up by such bundles of wool will provide sustenance to the plant lest it dry out by the suction of the sun. This done, you will surround the flower with a circular hour-scale, arranged with the hours divided according to the latitude of your region, so that the centre of the flower fitted with the pointer F can indicate the hours on it. Thus when the sun rises you will expose the mechanism to the open sky, and