  Various recent experiments hint at a geometry dependence of scaling relations
in Rayleigh-B\'enard convection. Aspect ratio and shape dependences have been
found. In this paper a mechanism is offered which can account for such
dependences. It is based on Prandtl's theory for laminar boundary layers and on
the conservation of volume flux of the large scale wind. The mechanism implies
the possibility of different thicknesses of the kinetic boundary layers at the
sidewalls and the top/bottom plates, just as experimentally found by Qiu and
Xia (Phys. Rev. E58, 486 (1998)), and also different $Ra$-scaling of the wind
measured over the plates and at the sidewalls. In the second part of the paper
a scaling argument for the velocity and temperature fluctuations in the bulk is
developeVarious recent experiments hint at a geometry dependence of scaling
relations in Rayleigh-Benard convection. Aspect ratio and shape dependences
have been found. In this paper a mechanism is offered which can account for
such dependences. It is based on Prandtl's theory for laminar boundary layers
and on the conservation of volume flux of the large scale wind. The mechanism
implies the possibility of different thicknesses of the kinetic boundary layers
at the sidewalls and the top/bottom plates, just as experimentally found by Qiu
and Xia (Phys. Rev. E58, 486 (1998)), and also different $Ra$-scaling of the
wind measured over the plates and at the sidewalls. In the second part of the
paper a scaling argument for the velocity and temperature fluctuations in the
bulk is develope
