Walsh may have pieced together some of the concertos from Handel's other compositions, but Handel himself did this as well, as did many composers of the time. The music used in the six concertos was written roughly between 1738 and 1751.
Handel wrote the concertos for a specific purpose; as entertainment for the audiences of his operas and oratorios during intermissions. These concertos were used as drawing cards, kind of an extra bonus to entice listeners to attend the opera or oratorio. An advertisement for a Handel oratorio concert that ran in the London Daily Post on the 5th of March 1735 made mention of the performance of two concertos:
At the Theatre-Royal in Covent Garden, this present Wednesday .... will be perform’d an Oratorio, call’d Aesther. With several New Additional Songs; likewise two new Concertos on the Organ.Handel was a virtuoso organist, and his powers of improvisation were put to good use, as he would sketch out the orchestral parts in score and improvise much of the organ part. The concerto is in the form of a sonata chiesa and has four movements:
Adagio - The first movement begins with the orchestra playing a slow, noble theme. The soloist enters and comments on the theme with the strings providing a light, occassional accompaniment.
Allegro - Handel follows the pattern of a sonata chiesa as the next movement is quicker of tempo and lighter in mood.
Adagio (Organo ad libitum) - Handel did not notate any music for this movement, but left the entire third movement to improvisations from the organist, which would have been Handel himself when these concertos were first played.
Allegro - A favorite of Handel's for he first used the tune in a concerto for violin over twenty years prior.