Good Eye: The best specimen is the roof

The roof-truss design provides unimpeded space for viewing the collections.

The Wagner Free Institute of Science is famed for its collection of animal and fossil specimens housed in rows of 19th-century glass cabinets. But one of the museum's greatest scientific works is neither animal nor vegetable. It's mineral: the patented iron-and-wood truss that holds up the roof.

The Wagner's main exhibit hall is such a spectacular sight it's doubtful many visitors have paid much attention to the structure overhead. Yet the roof truss is what makes that immense, open-plan room possible.

Founded by William Wagner in the 1850s, when North Philadelphia was a patchwork of rural estates, the museum is the product of the 19th-century obsession with cataloging the natural world. In his job as an executive for shipping magnate Stephen Girard, Wagner was able to travel the world and assemble a huge collection of specimens, including dinosaur bones and the remains of a saber-tooth tiger. In 1859, he commissioned John McArthur Jr. (soon to be the architect of City Hall) to design a home for his booty.

Two blocks west of Temple University's Liacouras Center, the Wagner is what's called a cabinet museum. Unlike today's museums, which put only their best holdings on display, the Wagner's entire collection was laid out in glass-topped cabinets so visitors could study the specimens while strolling the aisles. The challenge was to design a roof that could span this 60-foot-wide hall without a lot of supporting columns cluttering up the space.

McArthur reached out to an engineering firm specializing in railroad truss bridges, Stone, Quigley & Burton. The firm devised a barrel-vault roof supported entirely by nine arched wooden trusses. Each is formed by a series of X-braces sandwiched between bowed, wooden slats and held in place by iron pins. The space is completely open except for the columns holding up the balconies. So unusual is the truss design the engineers had it patented when the museum opened in 1865.

The trusses are still in good condition, but the roof and the plaster ceiling are not. In preparation for a major renovation, the Wagner has installed a temporary ceiling over the exhibit room. But on Wednesday, the Wagner will kick off its 150th anniversary - and the start of a $15 million fund-raising effort - with a party and lecture by the Athenaeum's Bruce Laverty. Once you've marveled at the displays and enjoyed a piece of cake, ask a guide for a peek at the trusses.



Four stops north of John McArthur's City Hall, the Wagner is accessible by the Broad Street subway. Exit at Cecil B. Moore Station, walk one block north to Montgomery Avenue, then head west to the Renaissance Revival building at 1700 W. Montgomery. For information, go to