Italian stringed keyboard instruments and simple geometry: Some new developments at the Russell Collection of Early Keyboard Instruments
Grant O’Brien
The octave spinet by Petrus Orlandus, 1710
Absolutely nothing is known about the place in which this spinet was made nor about the maker of the tiny octave spinet signed “Petrus Michael Orlandus 1710” in the Russell Collection. The instrument is of integral construction, it has a beautiful layered rose in red and white parchment with lower tiered decorations in gold. The compass is C/E to c3 and the scalings are clearly intended for iron stringing at octave pitch.
Figure 4 - Plan view soundboard photograph
Octave spinet by Petrus Michael Orlandus, 1710
Russell Collection of Early Keyboard Instruments, Inv. no. S2-PO1710.11
Here again the measurements of the baseboard not including the case sides and the other measurements of the case, keyboard and string scalings are revealing. The top part of Figure 5 below shows the measurements of the baseboard of the Orlandus spinet without the case sides measured in mm. Here a clear relationship, between the tangent of the angle of the sloping rear side, and the measurements of the rest of the case sides is, admittedly, not at first obvious. However, a careful analysis shows that the relationship that appears to work for all of the measurements of the baseboard and for the measurements of the case height, keyplank, the string scalings and the other measurements of the instrument is:
tan 17º = 0.306 
= 0.303
= 0.303 = tan 16.84º
This in turn
suggests that the length of the unit of measurement being used is close to 
= 21.55 mm or 
= 21.50mm[13],
both of which are close to the length of the oncia that was used in Sicily in and therefore probably in
Palermo. Indeed this is not
surprising. The type of elaborate
birthday-cake rosette in many layers is a feature commonly found in instruments
made in this region of Italy as are a number of the other features of the
spinet. Hence it appears that Petrus
Michael Orlandus (Pietro Michele Orlando) was almost certainly working in
Palermo when he built this instrument.
Figure 5 - Measurements of the baseboard without the case sides in mm (above)
and in units of the Palermo oncia = 21.483mm (below)
Octave spinet by Petrus Michael Orlandus, 1710
Russell Collection of Early Keyboard Instruments, Inv. no. S2-PO1710.11
Footnote:
[13] The authors Ludovico Eusebio, Compendio di Metrologia Universale e Vocabolario Metrologico, (Unione Tipografico Editrice Torinese, Turin, 1899; reprint by Forni Editore, Bologna, 1967), L. Malvasi, La metrologia italiana ne' suoi cambievoli rapporti desunti dal confronto col sistema metrico-decimale, (Fratelli Malvasi, Modena, 1842-44), and Angelo Martini, Manuale di metrologia, (E. Loescher, Turin, 1883; reprint Editrice Edizioni Romane d’Arte, Rome, 1976) all give a value of 257.8mm for the Sicilian palmo which is known to have been divided into 12 once. Therefore one oncia had a length of 257.8/12 = 21.483mm. Clearly this is very close to the value of the oncia determined here.