Some of the mathematicians have demonstrated by formulæ, unsupported by facts, that there is a considerable amount of skin friction to be considered, but as no two agree on this or any other subject, some not agreeing to-day with what they wrote a year ago, I think we might put down all of their results, add them together, and then divide by the number of mathematicians, and thus find the average coefficient of error. When we subject this question to experimental test, we find that nearly all of the mathematicians are radically wrong, Professor Langley, of course, excepted. I made an aeroplane of hard rolled brass, 20 gauge; it was 1 foot wide and dead smooth on both sides; I gave it a curvature of about 1/16 inch and filed the edges, thin and sharp. I mounted this with a great deal of care in a perfectly horizontal blast of air of 40 miles an hour. When this aeroplane was placed at any angle between 1 in 8 and 1 in 20, the lifting effect was always just in proportion to its angle. The distance that the front edge was raised above the horizontal, as compared with the width of the aeroplane, was always identical with the drift as compared with the lift. On account of the jarring effect caused by the rotation of the screws that produced the air blast, we might consider that all of the articulated joints about the weighing device were absolutely frictionless, as the jar would cause them to settle into the proper position quite irrespective of friction. I was, therefore, able to observe very carefully, the lift and the drift. As an example of how these experiments were conducted, I would say that the engine employed was provided with a very sensitive and accurate governor; the power transmission was also quite reliable. Before making these tests, the apparatus was tested as regards the drift, without any aeroplane in position, and with weights applied that would just balance any effect that the wind might have on everything except the aeroplane. The aeroplane was then put in position and the other system of weights applied until it exactly balanced, all the levers being rapped in order to eliminate the friction in their joints. The engine was then started and weights applied just sufficient to counterbalance the lifting effect of the aeroplane, and other weights applied to exactly balance the drift or the tendency to travel with the wind. In this way, I was able to ascertain, with a great degree of accuracy, the relative difference between the lift and drift. If there had been any skin friction, even to the extent of 2 per cent., it would have been detected. This brass aeroplane was tested at various angles, and always gave the same results, but of course I could not use thick brass aeroplanes on a flying machine; it was necessary for me to seek something much lighter. I therefore conducted experiments with other materials, the results of which are given. However, with a well-made wooden aeroplane 1 foot wide