"Spanning the Lagan, now we have in view
The great Long Bridge, with arches twenty-two"
to which he appended a note-- "It has only twenty-one, but as a poetical licence, and for the sake of rhyme, I had to add another arch"
from: Guide to Belfast: The Giant's Causeway, and the Adjoining Districts of the Counties of Antrim and Down ... By William McComb 1861
An old print of the Long Bridge.