Wednesday, February 16, 2011

The Science Behind Philadelphia’s City Squares

Posted by Paul Halpern on 02/16 at 10:08 PM

Philadelphia is a city rich with scientific history.  Of the four main squares in its downtown area, it is interesting to note that three are named after people who have had at least some association with science:  James Logan, Benjamin Franklin, and David Rittenhouse.  The fourth is named after George Washington, who was a surveyor by occupation early on in his career, an endeavor based on detailed knowledge of mathematics.

Logan was secretary to William Penn when Pennsylvania was founded.  He was a lover of books and interested in all manner of scholarly topics, including physics. He used his substantial wealth to fund scientific endeavors.  For example, he supported Thomas Godfrey, another notable Philadelphian who was one of the inventors of the sextant.  In 1709, Logan purchased a copy of the first edition of Newton’s Principia; he would later acquire copies of the second and third editions as well.  Thus Logan played a pivotal role in introducing Newton’s work to the colonies.  During a 1710 trip to London, Logan witnessed Newton performing an experiment before an audience at St. Paul’s Cathedral.

Franklin is, of course, well known for his pivotal contributions to the physics of electricity.  He was fortunate to have made the acquaintance of Peter Collinson, a prominent London merchant and member of the Royal Society, who provided books, plants, animals and scientific supplies to the Colonies.  In 1746, thinking that the brilliant Franklin might be interested in the wonders of electricity, Collinson sent him a package that included a glass tube used for electrostatic experiments and an article by Swiss professor Albrecht von Haller that described what was then known about the field.  Making great use of the tube and the article, Franklin embarked on an intensive, exploration of the nature of electrical properties.  During the late 1740s, Franklin reported to Collinson a number of spectacular insights about electricity, including defining the concept of positive and negative charge, and establishing that electrical attraction and repulsion can act over a distance rather than just through contact. 

Rittenhouse made pivotal contributions to optics and astronomy.  Adept at building mechanical devices, he set out in 1767 to build an orrery:  a machine replicating the motions of the planets and moons in the Solar System using Kepler’s laws as a guide. In 1769 he investigated the transit of Venus by constructing a modified refracting telescope to measure the exact time of the transit.  The goal was to use observations of the transit around the world to obtain the distance from Earth to the Sun through the method of parallax.  In the 1780s he constructed the first diffraction grating.  By placing fine hairs parallel to each other, he produced a grating with about 250 lines per inch, and created a distinct interference pattern with six bright spectral lines on either side.  Borrowing optical instruments from Franklin, including a prismatic telescope and micrometer, he measured the angle to each spectral line and discovered integer ratios between the higher order and first order lines. In the 1790s, he investigated the effects of air density on pendulum irregularity, and designed a suspended pendulum that was balanced such the buoyant forces on either side cancelled out.

Clearly, it is hard to wander around downtown Philadelphia without stumbling upon science history—even in the names of its principal squares.

(Parts of this post are excerpted from my article, “Life, Liberty and the Pursuit of Physics:  The Physical Tourist Visits Philadelphia,” Physics in Perspective, Vol. 11, No. 2, 2009)

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