Photo Credit: RovingRube
02 October 11

"The tower crane [ Alt View 1] grows in height ... by adding new sections to itself without being dismantled ... The jumping frame assembly [thicker section at the top in Alt View 2] with its two large hydraulic jacks is the key ... [they] can raise the entire upper structure of the crane about 14 feet, providing the opening for the new tower section." (from The Building of Manhattan, by Donald A. Mackay)

Tower crane extension section
Tower crane extension section, East 51st Street(?) 
Full Size Image Restore Image Alt view 1 Alt view 2