Above are two versions of the Horizon tool invented and described by Leon Battista Alberti to allow a man of average intellect to accurately draw the map of Rome.
The tools have been cut to a 15″ diameter to allow for a decently scaled map. A second Spokeless version of the toll was created for more accurate plotting (as opposed to the previous version being interrupted with spokes).
After having read Alberti’s description of the construction the “Horizon” tool it becomes clear that this tool is made only to replicate his initial surveying studies. There is some complexity when he begins to differentiated a corner from an apex, consequentially suggesting different drawing rules when constructing the lines through the co-ordinates.
I would imagine that because the grid used to initially survey City Walls was a radial one as opposed to an orthogonal grid, that it would have been quite difficult to locate the co-ordinates (possibly having to return to the Capitol to count the number of steps for every important corner, apex or gate).
An Astrolabe used to treat the Humours
The astrolabe above can be used find the location of the stars above the person in question at his/her place and time of birth. This tool became useful to doctors attempting to treat or balance a patients emotional instabilities. By reading the stars (zodiac) that blanketed the sky at the time and place of their birth the medical doctor could prescribe counter measures to balance out the temperaments of the four Humours that people are influenced by.
How to use the Astrolabe
Use the back of the astrolabe to find the date in question in the calendar. Move the label round until it lines up with this date. Then read along the line of the label to see the position in the Zodiac calendar. On the front of the Astrolabe, line up the rule with the Zodiac unit found on the rete. Holding both rule at rete (fixed together), rotate the rete until the rule reaches the time of birth of the patient.
Having followed these instructions, all the stars that fall within the horizon line should be the project of stars at the time of the patients birth. Provide Prescription accordingly.