() SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. -I 



Simultaneously with these, original experiments with the whirling-table were 

 being conducted along lines of research, which though necessary have only been 

 indicated. We have, (hen, at leas! five subjects, so distinct that they can only 

 lie properly treated separately, and accordingly they will he found in Chapters 

 VI I, VI!!, IX and X, and in Pari Third [in preparation |. 



It is inevitable that in so complex a study some repetition should present 

 itself, especially in the narrative form chosen as the best method of presenting 

 the subject to the reader. Each of these chapters, then, will contain its own 

 historical account of its own theme, so that each subject can be pursued continu- 

 ously in the order of its actual development, while, since they were all interde 

 pendent and were actually going on simultaneously, the order of dales which is 

 followed in each chapter will be a simple ami sufficient method of reference from 

 one to the other. 



Experiments with Small Models 



In order to understand how the need arises for such experiments in fixing 

 conditions which it might appear were already determined in the work " Experi- 

 ments in Aerodynamics," 1 it is to be constantly borne in mind, as a considera- 

 tion of the first importance, that the latter experiments, being conducted with 

 the whirling-table, force the model to move in horizontal flight and at a constant 

 angle. Now these are ideal conditions, as they avoid such practical difficulties 

 as maintaining equilibrium and horizontality, and for this reason alone give 

 results more favorable than are to be expected in free flight. 



Besides this, the values given in " Aerodynamics " were obtained with 

 rigid surfaces, and these surfaces themselves were small and therefore manage- 

 able, while larger surfaces, such as are used in actual flight, would need to be 

 stiffened by guys and like means, which offer resistance to the air and still 

 further reduce the results obtained. It is, therefore, fairly certain, that noth- 

 ing like the lift of 200 pounds to the horse-power for a rate of 40 miles an houiy 

 obtained under these ideal conditions with the whirling-table, will be obtained 

 in actual flight, at least with plane wings. 



The data in " Aerodynamics " were, then, insufficient to determine the con- 

 ditions of free flight, not alone because the apparatus compels the planes to move 

 iii horizontal flight, but because other ideally perfect conditions are obtained by 

 surfaces rigidly attached to the whirling-table so as to present an angle to the 

 wind of advance which is invariable during the course of the experiment, whereas 



the surfaces employed in actual llighi may evidently change this angle and cause 



Ml is desirable that tin- reader should he acquainted with the contents of this treatise, and of 

 another by me, entitled "The Internal Work of the Wind," both published by the Smithsonian Insti- 

 tution. A knowledge of these works is not ahsolutely necessary, hut of advantage in connection with 

 what follows. 



2 " Experiments in Aerodynamics," p. 107. 



