Probably by a water heater fuelled with lumber scraps and hog fuel and then emptied into a pond lined with clay. One way was to use the exhaust steam from the mill engines to heat water. These were first used about 1890. Sometimes a roof covered the pond to keep the heat in. These ponds would be roughly 100-125 feet by 300-500 feet. Today hot ponds are usually made of reinforced concrete, to prevent polluting streams and ground water. Fraser's, in Plaster Rock, started about 1900 but did not build a hot pond until 1953. I guess they cut timber in winter and sawed in the summer using the same crew.
The hot pond in Juniper appears to have been heated by steam as John Brown had a 'scald over the whole body'. He fell into the pond and apparently lived for 10 hours after the accident. There appears to have been a 'special file' (inquest?). He lived in Bath and the doctor was from Plaster Rock; kind of strange since he was injured in Juniper. Also strange is that he died at 10 a.m., suggesting he fell in about midnight, if he lasted 10 hours.
Thomas.