[idunning]

Varies wildly depending on field even within an institution, and is still only for 9 months of the year (pay for the summer either with grants, consulting, or by teaching summer school classes [if there are any]).

[Fomite]

While a historian (like the author) might not get paid summer salary, this is not generalizable.

[antognini]

My professors in the astronomy department at my university (a large state school) don't get paid over the summer either.

[Fomite]

It very much depends on the department. For example, most NSF & State supported departments (like astronomy) draw a 9-month salary. Many if not most NIH funded positions, like those in medical schools or schools of public health, are 12-month positions. There are lots of those positions, they're just somewhat less visible.