If the patch were put on the outside, the action would still be on the plate, which would in time be worn through, then the pressure of the steam would force the water between the plate and the patch, and so corrode it; and during a jerk or extra pressure, the patch might be blown off.
It is on the same principle that mud-hole doors are on the inside.
If you found several thin places, what would you do?
Patch each, and reduce the pressure.
If you found a blistered plate?
Put a patch on the fire side.
If you found a plate at the bottom buckled?
Put a stay through the centre of the buckle.
If you found several?
Stay each, and reduce the pressure.
The crown of the furnace down?
Put a stay through the middle, and a dog across the top.
If a length of the crown were down, put a series of stays and dogs.
A cracked plate?
Drill a hole at each end of the crack; caulk the crack, or put a patch over it.
If the water in the boiler is suffered to get too low, what may be the consequence?
Burn the top of the combustion chamber and the tubes; perhaps cause an explosion.