During the time the North Bloomfield Gravel Mining Company operated, as many as eight monitors were in use at the same time. Fashioned after Civil War cannons, the large monitors could weigh as much as 1 1/2 tons. The large monitors in the Diggins were capable of using 25 million gallons of water in a 24 hour period or over one million gallons an hour. The wooden box toward the rear of the monitor was loaded with rock to raise the barrel of the monitor and act as a counter balance created by the bucking water pressure leaving the nozzle.
The blasting power of a monitor or water cannon came from elevation drop alone. No mechanical devices were used. The water that came from a nearby resovoir exited in large pipes then graduated down in size until they reached the monitor and through a 10 inch nozzle. A large monitor would blast water at approximately 5,000 pounds per square inch, enough pressure to move a boulder the size of a small car. Different sizes of monitors had various functions. Large monitors were used to bring down the mountain, while small monitors were used to keep the debris moving down the sluice or long toms used to collect the gold and then on to the final exit point.
The miner that operated the monitor was known at the “piper.” He was paid the most for he had to know how to operate that big monster properly. If he didn’t, cave-ins occurred catching men unprepared thus causing injury and even death.
Legend has it that a miner with a dirty shovel set his tool into the stream of the water exiting from the cannon and the force of the water against the shovel moved the monitor’s aim with the greatest of ease and thus led to the invention of the ball and socket design we know today.
Monitors were made at the Joshua Hendy and the Parke and Lacy Company in San Francisco. Also monitors and hydraulic equipment were made locally in the Nevada City Foundry.