I moved to the city a couple of years ago, and have noticed how clean the roofs always are. No leafs get up there. Further, from every roof there runs a pipe all the way along the edges of the roof, and to the ground in order to make the rain that ends on the roof go to the ground without pouring do...

After just 2 months a windmill has gone from white to grey. - This is a problem; the wings on a windmill have to better, and more aerodynamcially constructed than even aeroplane wings, since they have to pointed directly at the wind; the particles that heap up on the wings and slowly degenerate the ...