Birdsill Holly Jr. (November 8, 1820 – April 27, 1894) was an American mechanical engineer and inventor of water hydraulics devices. He is known for inventing mechanical devices that improved city water systems and patented an improved fire hydrant that is similar to those used currently for firefighting. Holly was a co-inventor of the Silsby steam fire engine. He founded the Holly Manufacturing Company that developed into the larger Holly Steam Combination Company that distributed heat from a central station and developed commercial district heating for cities in the United States and Canada.