There are a number of drawings from the Brunel era on display in the ICE library just now. David Greenfield raised a question about this on CEHX. The pictures can't be displayed there so here they are.
Looking at the full picture first, the centre is shown in two halves with a break line jst to the left of the mid span. The right hand section is supported on 3 pies such that 5 would be used for the full span. There is a branching tree support from each pile with folding wedges for adjustment and decentring. This section is drawn in beautiful detail and colour washed.
The left hand section is shown plain but also carries dimensions. I think the dimensions are in a different hand so they raise a number of questions, see below. Also, the left had section has 2 piles for support and is set out so that 4 are needed for the full span. ie there would be four trees rather than three and two bits. Also the base of this frame is much higher.
One question, then, is, is this a drawing showing two alternative centres, or were the two frames used side by side so that 9 plies appear to obstruct flow and traffic in the river?
The dimensions are also interesting. I used Excel to convert them to decimals and plot them. At about the quarter point, the resulting line is about 9in below the semi-ellipse that passes through the springings and the crown. So, is the arch not semi-elliptical at all? Did brunel set out a cusp at the crown? The dimension lines have no arrows to indicate where they are measured from and to. It may be that there is say 4in of lagging above this profile. that would mean that Brunel expected a 7in drop at the crown to bring the rise to 24ft (which sounds more reasonable than 24ft3in). That would also imply a slight rise at the quarter point. I haven't checked that out yet, but it seems to hang together.
The list below gives the setting out dimensions and the elliptical equivalents for the half span.