The cross sections have side length equal to the vertical distance between y = √x and y = 1, or |√x - 1|. The two curves meet at the point (1, 1), and y = √x meets x = 4 at (4, 2), so we'll be integrating with respect to x on the interval [1, 4]. Over this interval, √x ≥ 1, so |√x - 1| = √x - 1.
A cross section of thickness ∆x has volume
(√x - 1)² ∆x = (x - 2√x + 1) ∆x
Then the volume of the solid is
[tex]\displaystyle \int_1^4 (x - 2\sqrt x + 1) \, dx = \boxed{\frac76}[/tex]
