The picture is sufficiently fuzzy and dark that I am not sure but that might be a packing follower and those cylinders use a string type of packing. IF SO then it is merely a matter of backing off the nuts and pulling out the follower and then adding a ring or two of packing.
Those are low pressure cylinders and they look old enough to be of that type.
If that has a bolt on rod gland cap you can remove the bolts slide the cap down and dig the old packing out with a screwdriver. Purchase some chevron style packing from an equipment dealer cut the individual pieces diagonally and slip them over the rod and shove them up in the cylinder head. Make sure and stagger the cuts just like putting piston rings on a piston. This is a quick field fix but will last quite awhile. Good luck!
But FIRST, just tighten the nuts a bit and see if it helps!
CS