Paolo Agostino (ca.1583-1629) was one of the great contrapuntists and composers of canon. Among various positions he held throughout his life as organist, or choir director, he eventually became the director of the choir at Cappella Giulia at the chapel of St. Peter at the Vatican, a post also held at other times by Palestrina and Domenico Scarlatti (see https://it.wikipedia.org/wiki/Cappella_Giulia#Direttori)
This canon is exemplary in its concision, use of embellishment, and invertible counterpoint. And it is a canon per tonos. "Canone artificioso, che cresce un Tuono." This enigmatic representation is my own, and I added the hints: sub diapason, diapente, sub diatessaron.
Each of the four voices repeats a full tone higher each time it returns. The ranges work out quite nicely, at least for the first several iterations, the four vocal ranges hitting their peaks at roughly the same time.
This canon will fall on different ears differently, and there is no avoiding the ascension of the tonality. However, there is careful artifice in the gradual and subtle introduction of the tonal elements that push this tonality upward.
The tonic note is the first note, heard offset, falling on the offbeat; it establishes the tonic as it falls, but its metric placement allows the next two entries to agree with the tonic harmony in both measures two and three. The first three notes of each entry dance around the note which is to be the newly-introduced, most sharp note, each entry either leading with or repeating such a carefully ordered tetrachord.
What is more, the new note falls after the fourth beat of the entry, that is to say after the downbeat, as the resolution of a suspension. providing the sweetness of the harmony as a nonchalant formality after the native portion of the harmony has planted its feet. The repetition has the effect of cementing the previously introduced scale degrees and confirming the modulation.
As the canonic strand repeats in all voices, each of its four strands serves at times in the role of baseline, melody or middle voice, and it should be clear from this that the canon works in perfect invertible counterpoint. Furthermore, the inversion is at the twelfth as can be seen when the 2-3 suspension on the downbeat of measure three inverts to a 4-3 suspension on the following downbeat.
These successes lead me to forgive the gratuitous rest nearly equivalent to a breve, which separates the canon from its repetition with sufficient time to form only triple counterpoint. In fact there is much to be appreciated here.
This canon is found on page 302 of Esemplare o sia saggio fondamentale pratico di contrappunto fugato, Part 2 by Giovanni Battista Martini (Bologna, 1775). On that page, Martini says of Agostini:
Quello, che trovasi di raro in tutte le Opera di questo celebre compositore, si e una chiarezza, una naturalezza, e pastosità singulari, e abbenchè egli abbia introdotti nella fae composizione gli Artifici più difficoltosi di Canoni, di tutte le Specie, e di Contrappunti doppi di tutte la forta, ciò non ostante egli ha saputo unire a tali Artifici, cosa molto rara, la chiarezza, e naturalezza tanto nella Melodia di ciascuna delle parti, che nell' Armonia formata dalla loro unione.
Paolo, Trovasi di raro.
What I find outstanding in all the works of this famous composer is a singular clarity, naturalness, and mellowness, and he has introduced in his composition the most difficult artifices of canons, of all types, and of double counterpoints, of all of it, nevertheless he has been able to unite these artifices, a very rare thing, the clarity, and naturalness both in the Melody of each of the parts, and in the harmony formed by their union.
This is very high praise, especially given the fact that the canon is introduced in Martini's chapter as an afterthought in the context of Martini's conclusive remarks on an eight-voice fugue by Agostini. (mistranslation, mine)