A couple of months ago, in a column about extremely big airplanes, I said that although a solid object gains weight in proportion to the cube of its linear dimensions, a hollow shell, like an airplane, gains weight, and surface area, in proportion to their square. A reader, C.L. Landers, formerly director of helicopter design at McDonnell Douglas, pointed out that this would be true only if the sizes of structural components such as skins and stringers did not increase. He's right; bigger airplanes, of course, have more massive structural components than small ones do. I should have thrown in some weasel words like "not always," because skins, in particular — which make up a large portion of structural weight — tend not to thicken quite in proportion to linear dimensions, but to acquire more and more internal stiffeners instead. In general, however, and particularly in the context of immense airplanes, Mr. Landers is right and I was wrong.