According to the folks at AdirondackChair.com, the original chairs were called Westport Chairs, named after the town near Lake Champlain where they were invented and patented. The patent holder and builder, Harry Bunnell, built his chairs from hemlock and painted them either green or dark brown. Today original Harry Bunnel-built Westport chairs command prices of over $1,000.
These days, however, many home woodworkers use clear white pine instead of hemlock when building a replica of Harry's first chair.
White pine has long been known for its utility by both woodworkers and carpenters. It's lightweight, close-grained and resists warping or checking. There is an extensive write-up of white pine's superior characteristics at the Craftsman Style website (see Resources). One of the wood's most notable qualities is its superior ability to take and hold paint, particularly enamel. As outdoor furniture is destined to spend most of its time outdoors, this is an important consideration when selecting the wood for an Adirondack chair.
The nice people at Minwax have free project plans available that will show you how to construct an Adirondack chair of your very own (see Resources). It includes a materials list, cutting and assembly instructions, suggested woodworking tips and even finishing recommendations.