Tim;
I published a similar setup in the October 2001 issue of Arborist News magazine. The only difference was that I used a Schwabisch instead of a Blake's and I cautioned that you should not use the French prusik for the climbing hitch.
I tried a number of hitches for both of the knots in this configuration (the actual climbing hitch and the sliding 'adjustment' hitch). In my experience there are a number of knots that could be safely used for the adjustment hitch. Whatever knot is used I think it is best if the adjustment knot is as short as possible. You can also use a number of different knots for the climbing hitch, but, as I said above, I don't think that you should use the French prusik. The Fp can be exteremely variable depending upon the length, diamater, construction, material and wear of the tail and the diameter, construction and wear of the climbing line. The problem is that the Fp does not grab reliably after it has been advanced by your hand or the slack tender. There are times that you have to hold the Fp after it has been advanced so that it contacts the climbing line and tightens up to hold you in place. This becomes difficult or even impossible when the climbing hitch is moved farther from the climber. The particular setup that you now use may work well, but if you change some component (eg. use a different split tail) the Fp may not grab reliably. Someone else who uses a similar setup (i.e. a Fp for the climbing hitch) may find that it does not grab at all.
People should also be aware that the adjustment knot (Blake's in your setup) may need some 'tweaking' as it is moved back and forth from the wider, spliced, part of the line (right near the eye) to the slightly smaller, narrower portion of the rope above the splice.
In general I thnk that an adjustable bridge is a great setup. I don't use it on every tree, but I do use it often; I like being able to quickly and easily adjust how far away the climbing hitch is. There are numerous situations when I want the climbing hitch close and within easy reach, but there are other times when I want the hitch farther away and self-advancing (I attach the slack tender to the other leg of the climbing line).
In that same AN article I also showed a 'non-detachable split tail' for a sliding bridge. Some years ago Knut Foppe introduced a double bridge. He did it as a 'double Distal', but it can be done with other hitches as well. A year or two ago Xian Hunter described an extendable bridge. All of these have been discussed, to a greater or lesser extent, on the ISA discussion board. Unfortunately, some of those posts have been archived and it may not be possible to retrieve them.
Mahk
