Those M&D words 4

[Monte Davis]

> "Gun-Purchase Arrangement", some kind of cannon supporting structure?
> 
> "The Carpenter of the Seahorse and his men have put up a 
> structure solid as 
> a Man o' War, tarr'd the Roof and all the joints, rigg'd a 
> couple of Blocks 
> in a Gun-Purchase Arrangement allowing the Gentlemen to slide 
> the Shutter 
> open and clos'd from inside. "

A man o' war had (1) moveable shutter panels closing its gun ports, and (2)
arrangements of ropes and pulleys (block & tackle) to absorb the gun's
recoil and haul it back into position. (When the latter failed, BTW, you had
a "loose cannon" -- a dangerouis juggernaut on the cramped, crowded gun
deck.)

Seems to me that here P. is combining the two for effect: literally, a
pulley system to open the roof of the telescope shelter for Obs;
metaphorically, a reminder (going back to the Seahorse-l'Grand battle) that
the Obs themselves may be volleys in an obscure war.