low water automatic mounted on the firewall at the right side of the boiler compartment

To the best of historian's knowledge there are no documented cases of a Stanley boiler ever exploding due to lack or water (or any other reason for that matter).  The earliest Stanleys were designed such that the boiler water level had to be controlled manually.  It was one of the tasks the driver had to remember to attend to as the car was being driven.  A sight glass on the dash provided visual indication of the water level in the boiler.  For cars built in 1916 and later a water automatic was installed to control the level of water in the boiler.

On early Stanley cars a short pipe was attached to the bottom of the boiler and was plugged with lead.  If the water got too low in the boiler and uncovered the end of the pipe which extended a short distance into the boiler, the lead would melt and the steam and water in the boiler would blast down into the burner usually extinguishing the fire.  For the non-condensing Stanley cars that had copper flued boilers, if the boiler ran low of water the heat melted the flues and the boiler simply leaked any remaining water and steam into the fire.  Thus, unlike a locomotive type boiler, running a vertical fire-tube boiler out of water doesn't present as much of an explosion hazard as it does with locomotive and other firebox boiler designs.

As the use of a lead filled pipe was prone to leaking and blowing out when there really wasn't a low water problem, a low water automatic was developed.  It continuously sensed the water level of the boiler and if the water level fell too low the automatic operated to close off the fuel supply to the main burner.

Located on the firewall, the low water automatic externally senses the water level at a given point near the bottom of the boiler.  When the automatic is cool due to water being in the expansion tube, the low water automatic allows fuel to pass to the main burner.  However if the boiler water level gets critically low then the low water automatic closes off fuel flow to the burner.  It is the success of the low water automatic that led the Stanley Twins to later design a similar device for controlling the actual water level in the boiler.

HOW THE LOW WATER AUTOMATIC PROTECTS THE BOILER

The Low Water Automatic works by the expansion of a brass tube that is connected between two steel rods.  The lower end of the expansion tube is connected to a loop of piping running between the bottom of the boiler and the top of the boiler.  The automatic is mounted vertically unlike the horizontal mounting of the Water Automatic.  The bottom of the low water automatic is also located a couple inches above the bottom of the boiler as to its connection point in the piping loop.   Whatever the water level is in the boiler, it will be equivalent to the water level in the loop of piping between the top and bottom of the boiler.

As long as the bottom end of the low water automatic is below the water level in the loop of piping between the top and bottom of the boiler there is no steam in the expansion tube regardless of the height of the water in the boiler.  However, if the boiler gets extremely low on water the water level in the loop of piping will drop below the connection point to the bottom of the low water automatic.  As soon as this happens any water in the low water automatic expansion tube runs into the boiler and the expansion tube fills with steam.  The steam causes the expansion tube to expand.

The lower end of the expansion tube is allowed to move against springs relative to the two steel rods of the low water automatic.  This allows the expansion tube to expand to any length and not apply undo forces to the valve stem or valve seat.   The valve end of the expansion tube is directly attached to the valve stem of a  valve mechanism that is plumbed into the main burner fuel line between the pumps and the steam automatic.  The valve is fixed to the upper ends of the two steel rods.  The steam in the expansion tube causes the valve stem of the valve assembly to block off the fuel flow to the main burner.  The result is the burner is shut down due to the low water level.

The low water automatic is mounted vertically on the firewall so that it doesn't respond to the radiated heat of the boiler and cause the expansion tube to expand when there is really enough water in the boiler.  However the low water automatic is close enough to the boiler in the way that it is plumbed and designed that it reacts rather quickly to changes in the boiler water level as determined by the piping loop between the top and bottom of the boiler.

When the boiler is sufficiently full of water such that the water level in the loop of piping  is above the height of the bottom of the low water automatic, the expansion tube is again cool as it contains no steam.  The expansion tube cools and returns to the length it started at.  This action relieves pressure on the valve stem and opens the pathway for main fuel to again flow to the burner.