The earliest baptismal fonts were designed for full immersion, and were often cross-shaped with steps (usually three, for the Trinity) leading down into them. Often such baptismal pools were located in a separate building, called a baptistery, near the entrance of the church. As infant baptism became more common, fonts became smaller. Denominations that believe only in baptism by full immersion tend to use the term “baptismal font” to refer to immersion tanks dedicated for that purpose.
The Masons Co, 
Kansas City, Missouri. … by Kansas City masonry, the Masons company, masonry repair Fountain restoration and historic masonry restoration. https://www.youtube.com/watch?v=lcKjYxCb_f0
Some fonts where water pumps, a natural spring, or gravity keeps the water moving to mimic the moving waters of a stream. This visual and audible image communicates a “living waters” aspect of baptism. Some church bodies use special holy water while others will use water straight out of the tap to fill the font. A special silver vessel called a ewer can be used to fill the font.https://www.youtube.com/watch?v=WkfhnWHMLSs&t=37s
The mode of a baptism at a font is usually one of sprinkling, pouring, washing, or dipping in keeping with the Koine Greek verb βαπτιζω. Βαπτιζω can also mean “immerse”, but most fonts are too small for that application. Some fonts are large enough to allow the immersion of infants, however.
Kansas City Craft Masons/Historic Masonry Company, Masonry, Fountain, Historic Preservation. 