The figure below shows two parallel lines, $ \ell$ and $ m$, that are distance $ 12$ apart:
[asy]unitsize(7);
  draw((-7, 0) -- (12, 0));
  draw((-7, 12) -- (12, 12));
  real r = 169 / 48;
  draw(circle((0, r), r));
  draw(circle((5, 12 - r), r));
  pair A = (0, 0);
  pair B = (5, 12);
  dot(A);
  dot(B);
  label("$A$", A, plain.S);
  label("$B$", B, plain.N);
  label("$\ell$", (12, 0), plain.E);
  label("$m$", (12, 12), plain.E);[/asy]
A circle is tangent to line $ \ell$ at point $ A$.  Another circle is tangent to line $ m$ at point $ B$.  The two circles are congruent and tangent to each other as shown.  The distance between $ A$ and $ B$ is $ 13$.  What is the radius of each circle?
1. **Identify the given information and set up the problem:**
   - Two parallel lines $\ell$ and $m$ are 12 units apart.
   - A circle is tangent to line $\ell$ at point $A$.
   - Another congruent circle is tangent to line $m$ at point $B$.
   - The two circles are tangent to each other.
   - The distance between points $A$ and $B$ is 13 units.

2. **Define the radius of each circle:**
   Let the radius of each circle be $r$.

3. **Determine the coordinates of the centers of the circles:**
   - The center of the circle tangent to $\ell$ at $A$ is $(0, r)$.
   - The center of the circle tangent to $m$ at $B$ is $(5, 12 - r)$.

4. **Calculate the distance between the centers of the circles:**
   Since the circles are tangent to each other, the distance between their centers is $2r$.

5. **Set up the distance formula:**
   The distance between the centers of the circles is given by:
   \[
   \sqrt{(5 - 0)^2 + ((12 - r) - r)^2} = 2r
   \]
   Simplify the equation:
   \[
   \sqrt{5^2 + (12 - 2r)^2} = 2r
   \]
   \[
   \sqrt{25 + (12 - 2r)^2} = 2r
   \]

6. **Square both sides to eliminate the square root:**
   \[
   25 + (12 - 2r)^2 = (2r)^2
   \]
   \[
   25 + 144 - 48r + 4r^2 = 4r^2
   \]
   \[
   169 - 48r = 4r^2 - 4r^2
   \]
   \[
   169 = 48r
   \]

7. **Solve for $r$:**
   \[
   r = \frac{169}{48}
   \]

The final answer is $\boxed{\frac{169}{48}}$