Richard Clark wrote:

The description you offer requires a porous plate which is absent in
every radiometer that has come down the pike

On the other hand, the absence of porous plates in operating
radiometers tends to cast some doubt on your claim that the plates
must be porous.

As for the local air, there is none in
many radiometers that are more sensitive than the Crookes.

Again, which radiometer? If you are arguing a "perfect" vacuum, then
like a free lunch, I would agree there's no such thing. The Crookes
radiometer requires a partial atmosphere to work, other radiometers
work quite fine with much less.

Depends entirely on what one intends to measure.

The distinction in the multiplicity of radiometers is significant and
relates to an effect predicted in Maxwell's work, and exhibited by
Lebedev; and Nichols, Tear, Hull, and Webb by THEIR radiometers. It
requires very little more work than stating the surface area and
rendering the expression of force in the units pascals if one wishes
to remain within their classical descriptions.

To put a number to it: 0.3 slug per sq. mile (under less than
demanding conditions)

If the twins can't cope with the crippling Newtonian math, then one
might grant them the mental prowess of scaling by area - something
within the reach of a very special fifth grader. Yet and all, this
bone that I am tossing them only answers the less than Extra special,
super duper credit question.

To keep it in pascals in conditions of sunlight, on the equator, at
noon, on the solstice: 3 µPa (one square meter is implied, but this is
certainly not the size of any radiometer's vane - hence this number
should be further reduced by roughly 1/10,000 or 300 pPa). If the
twins can follow the mathematical progression in the parenthetical
statement, they should be able to answer the less than Extra special,
super duper credit question.

There are two possible mechanisms here. One is a momentum exchange
effect which is orders of magnitude smaller than the other and can
only be observed at ultra high vacuum. The other (the one Denny
accurately described) is a radiative molecular heating effect which
creates a differential pressure and as such, can only be observed in
at most a partial vacuum. The way to recognize the difference is that
the two mechanisms produce their respective forces in opposite
directions.

73, ac6xg