Tetrahedral Principle in Kite Structure 221 



States were established a few years ago 

 equipped with the Marvin kite. 



Continuous meteorological observa- 

 tions at a great elevation have been made 

 at the Blue Hill Observatory in Massa- 

 chusetts, and Mr Rotch has demon- 

 strated the possibility of towing kites at 

 sea by means of steam vessels so as to 

 secure a continuous line of observations 

 all the way across the Atlantic. 



HARGRAVE'S BOX KITE 



Hargrave introduced what is known 

 as the " cellular construction of kites." 

 He constructed kites composed of many 

 cells, but found no substantial improve- 

 ment in many cells over two alone ; and 

 a kite composed of two rectangular cells 



FIG. I— HARGRAVE BOX KITE 



separated by a considerable space is now 

 universally known as ' ' the Hargave box 

 kite. ' ' This represents, in my opinion, 

 the high-water mark of progress in the 

 nineteenth century ; and this form of 

 kite forms the starting point for my 

 own researches (Fig. i). 



The front and rear cells are connected 

 together by a framework, so that a con- 

 siderable space is left between them. 

 This space is an essential feature of the 

 kite : upon it depends the fore and aft 

 stability of the kite. The greater the 

 space, the more stable is the equilibrium 

 of the kite in a fore and aft direction, 

 the more it tends to assume a horizontal 

 position in the air, and the less it tends 

 to dive or pitch like a vessel in a rough 

 sea. Pitching motions or oscillations 

 are almost entirely suppressed when the 

 space between the cells is large. 



Each cell is provided with vertical 

 sides ; and these again seem to be essen- 

 tial elements of the kite contributing to 

 lateral stability. The greater the ex- 

 tent of the vertical sides, the greater is 

 the stability in the lateral direction, 

 and the less tendency has the kite to 

 roll, or move from side to side, or turn 

 over in the air. 



In the foregoing drawing I have shown 

 only necessary details of construction, 

 with just sufficient framework to hold 

 the cells together. 



It is obvious that a kite constructed as 

 shown in Fig. i is a very flimsy affair. 

 It requires additions to the framework of 

 various sorts to give it sufficient strength 

 to hold the aeroplane surfaces in their 

 proper relative positions and prevent dis- 

 tortion, or bending or twisting of the 

 kite frame under the action of the wind. 



Unfortunately the additions required 

 to give rigidity to the framework all 

 detract from the efficiency of the kite : 

 First, by rendering the kite heavier, so 

 that the ratio of weight to surface is in- 

 creased ; and, secondly, by increasing 

 the head resistance of the kite. The 

 interior bracing advisable in order to 

 preserve the cells from distortion comes 

 in the way of the wind, thus adding to 

 the drift of the kite without contrib- 

 uting to the lift. 



fig. 2 



A rectangular cell like A (Fig. 2) is 

 structurally, weak, as can readily be 

 demonstrated by the little force required 

 to distort it into the form shown at B. 

 In order to remedy this weakness, inter- 

 nal bracing is advisable of the character 

 shown at C. 



This internal bracing, even if made of 

 the finest wire, so as to be insignificant 

 in weight, all comes in the way of the 



