There are many commonalities in theme between the “engine” and the Trevi Fountain. This makes it clear that Salvi had the design in mind, or even in front of him, on two different occasions. The first time was in 1728 as he was creating the “engine”, and the second was during the competition for the Trevi Fountain in 1732. We assume that the design we are studying was probably Salvi’s and was copied for Salvi from the original before 1721. The fountain Salvi created was so close to that of his hero’s because of all the five designs, and the wooden model he created, the architecture follows the style of Bernini perfectly. It seems unnecessary at this point to even attempt to further argue that the entire Salvi fountain design, besides a few needed adjustments and variation, was the same on creates by Sian Lorenzo Bernini.
The factors that are exactly the same include the same Corinthian columns flanking the identical large niche decorated with festoons; the same windows (even if they are arranged in a different pattern), with the identical little balustrade alternating with Corinthian pilasters; the crowned statues, some spaced out along the wings, some grouped in the central section. One interesting factor is that in addition to the motif of two bas-reliefs over the two niches caused Salvi not be satisfied with the limited space of the three windows on each side. Staying true to his model he wanted to expand his façade vertically on both sides of the fountain. He wanted another two tiers of windows which alternated with another three pilasters, just as the engraving in the wooden model that is kept in the Museo di Roma shows. This describes the fountain he wanted to build, and most of which he actually did.
So the next issue of debate, although many others have argued this, is the beautiful rocky juts, missing from Bernini’s second design of the fountain, detailed with plants and bushes, and even snaked, which goes further to imitate the Four Rivers Fountain.