362 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



the nitrous test shows the degree of phlogistication of air, and that only; 

 whereas our sense of smelling cannot be considered as any test of its phlogisti- 

 cation, as there are many ways of phlogisticating air without imparting much 

 smell to it; and I believe there are many strong smelling substances which do 

 not sensibly phlogisticate it. 



IX. Experiments on the Resistance of the Air. By Richard Lovell Edgworth, 



Esq., F.R.S. p. 136. 



Many experiments have been tried to ascertain the force and velocity of the 

 wind, with a view to the construction and management of different engines, and 

 more particularly for the purposes of navigation. Several machines, which have 

 been employed in these inquiries, are described in the Transactions of the r. s., 

 and in the memoirs of foreign academies; but the most accurate that I have seen 

 was invented by the late Sir Charles Knowles; and from a number of experiments 

 made with it, he had constructed tables, showing at one view the force of the 

 wind on each sail of a ship at every degree of velocity, from 1 to 90 miles an 

 hour. But these calculations, and many more of a similar nature, that are to 

 be met with in Belidor's Architecture Hydraulique, and other books, are founded 

 on a supposition that the effect of the wind is directly as the surface on which 

 it acts. If, for instance, its force be estimated as l on one square yard, its force 

 on 2 square yards should be estimated as 2, on 3 square yards as 3, &c; but in 

 fact this proportion is not to be depended on, nor must the resistance of surfaces 

 be estimated merely by their extent; as several other circumstances must be 

 taken into consideration. 



No figures can resemble each other more than a parallelogram and a square 

 having the same superficial contents, as they are both bounded by 4 straight lines 

 meeting at right angles, yet they oppose different degrees of resistance to the air. 

 If 2 similar cards, for instance, be placed opposite the wind, one on its end, 

 and the other on its side, and both inclined to the same angle, the wind will 

 have the greater effect on the card that is placed edgewise. To determine the 

 difference of resistance between these two surfaces, and to ascertain the effect of 

 other figures moving through the air, I tried the following experiments. The 

 first 2 are to be found in Mr. Robins's Treatise on Gunnery; but I thought it 

 proper to repeat them, that they might be more readily compared with others 

 made with the same apparatus, especially as Mr. Robins made use of a machine 

 constructed on a smaller scale than mine, and turning on friction wheels, which 

 are not proper for machines of this nature, nor indeed for any purpose, where a 

 uniform motion is required. 



Having fastened a strong joist of wood from one side of a large room to the 

 other, so as to form a kind of bridge at some distance from the floor, I erected 



