\section{Introduction}

If we could somehow view our galaxy from a distance, we could observe
the full beauty of its spiral structure directly.  From our vantage
point from within one of its sprial arms, however, we must go through
more trouble to determine the shape and structure of our galaxy.
Optical methods are largely ineffective at large distances.  The
advent of spectroscopy allowed us greater ability to measure distances
and speeds of faraway stars.  However, much of the visible light
emitted from stars is absorbed by the clouds of dust and gas that make
up the interstellular medium.  In addition, a lot of the galaxy is too
cold to emit visible light.
\par
It was Dutch astronomer H. Van de Hulst who first explored the
possibility of observing the universe using the radio spectrum.  He
theorized that spin-flip transitions in hydrogen would cause emissions
at the 21 centemeter wavelength in the radio spectrum. In 1951, these
lines were observed by three separate groups: Ewing and Percell at
Harvard, Christiansen at Sydney, and Muller and Oort in the
Netherlands.  The procedure outlined in the paper published by Van de
Hulst, Mueller, and Oort in 1954 proved the basis for the experiment
we performed in lab.
\par
Hydrogen is the most abundant element in our galaxy, and conditions in
the interstellar medium favor the interactions that produce the 21~cm
line. 21~cm radio emissions are not obscured by dust as are emissions
in the visible wavelength.  We see the entire galaxy at 21~cm, making
21~cm radio observations an important tool in the study of our galaxy.

