Fig. 8, ‹w›(‹a› + ‹z›)−‹w›(0.66(‹z› + ‹b›) + ‹b›−‹a›) = 0.33‹w›‹m› + 2‹w›‹a›, ∴ ‹z› = ‹m› + 5‹b›. Knowing ‹z›, we readily find that the new distance from ‹CP›_{fw} to ‹CG›_1 equals: 0.66(‹z› + ‹b›) + ‹b› = 0.66‹m› + 5‹b›. In a similar manner we may calculate the proper relative positions of the front and rear wings when we wish to move the center of pressure backward a distance, ‹b›, from the original ‹CP›_1 without changing the position of ‹CG›_1. From Fig. 7, we have as before: ‹w›(‹m› + ‹a›)−‹w›(0.66‹m›−‹a›) = constant, 0.33‹w›‹m› + 2‹w›‹a› = constant. Fig. 9, ‹w›(‹z›_1 + ‹a›)−‹w›(0.66(‹z›_1−‹b›)−‹b›−‹a›) = 0.33‹w›‹m› + 2‹w›‹a›. ∴ ‹z›_1 = ‹m›−5‹b›. Similarly we have for the new distance from ‹CP›_{fw} to ‹CG›_1: 0.66(‹z›_1−‹b›)−‹b› = 0.66‹m›−5‹b›. [26] It is to be remembered that these aerodromes were under incessant modifications, No. 4 for instance, presenting successive changes which made of it in reality a number of different machines, one merging by constant alterations into the other, though it still went under the same name. After 1895 the type of the models remained relatively constant, but during the first five years of the work, constructions equal to the original building of at least eight or ten independent aerodromes were made. [27] Chapter V. [28] “Pocketing” is a form of distortion in which the canvas or silk bags locally in numerous places between the cross-ribs. [29] The site of these experiments, which was 30 miles below Washington, has been described. The writer is designated by the initial “L”; Dr. Barus, who several times assisted, by the letter “B”; Mr Reed, carpenter, by “R”; Mr Maltby, machinist, by “M”; and Mr. Gaertner, instrument maker, by “G.” [30] Weights and dimensions are here given in approximate pounds and feet. [31] “Experiments in Aerodynamics.” [32] On the data of “Aerodynamics,” a plane having 1.8 sq. ft. of surface per pound, and advancing at an angle of 20°, would soar at a speed of 24.1 ft. per second. [33] It will be remembered that the purely theoretical conclusions just cited apply to the power delivered in direct thrust, but that of the above actual H. P. an indefinite amount was lost in friction and slip of propellers. [34] It may be observed that at this time the position of the ‹CP› was calculated on the assumption that the pressure for flight surfaces was proportional to the areas, without also allowing for the fact that the following surfaces, like the tail, were under the “lee” of the wind and so far less efficient. It follows, then, that the value ‹CP›−‹CG› was not really 0, as was assumed, but something considerable. [35] Very exact accuracy in these minute details is indispensable to the efficient working of the engines. [36] The reader who may care to note the evolution of this boiler, by trial and error, will find a portion of the many discarded types shown in Plate 13. [p123]