The Industrial Revolution drove the modern city outward and upward--outward, as the population grew; upward because the financial and administrative heart of the city had nowhere else to go. The most impressive, or most obvious, manifestation of that growth was the skyscraper, which began to poke into the clouds in U.S. cities in the late 19th century. It was an innovation that relied on several important inventions for its success.
Taking the Weight Off
What holds a structure up? Until the 19th century, the answer was "the walls." That meant thick walls and a wide base of heavy stone at the bottom, supporting smaller and smaller upper decks, like the Pyramids. It also meant heavy braces to keep the walls from collapsing under the weight of the roof, like the flying buttresses on medieval Gothic cathedrals. By the 19th century, however, steel had come into its own as a construction material. Steel was much stronger per pound than stone, and engineers learned to transfer the weight loads from the walls to the frame, allowing greater heights on smaller building lots. Once the "curtain walls" were cladding, not support, they could be opened up with glass windows to provide light and air.
Penthouse Floor, Please
Moving people and goods vertically takes effort--mechanical effort. The taller a structure, the greater the need for a quick way to go up and down. Anyone who has climbed stairs for exercise or during a power blackout may appreciate, or curse, the work of 19th-century innovator Elisha Gray Otis and his safety hoisting device, which showed a jittery public how to step into a box and be rapidly lofted into the heavens--and then rapidly yet safely set down again.
Moving in Circles
Theophilus Van Kannel's revolving door patent of 1888 solved two vexing problems confronting big buildings. Swing doors constantly open and shut as hundreds or thousands of people arrive and depart, causing the building's heat (or air conditioning) to be lost to the street. When entering or leaving a big building through a swinging door when the outside and inside air pressure are changing, sometimes the door wildly flaps: other times, the door turns into an immovable airlock. The revolving door solved those problems, and if balanced properly, the door could be moved with the shove of a hand.
To Build Up, Build Down
Skyscrapers need to be anchored into bedrock; a mere stone or concrete foundation won't do. While construction steel can support great weight, it still weighs a lot itself. Construction equipment to excavate foundation holes, then drill and blast into bedrock to set steel beams and pour concrete, became available in the 19th century. Tower cranes, tall enough to raise prefabricated steel into place yet light enough to be piggybacked upward as the building grew, proliferated.