VOL. LVIII.] PHILOSOPHICAL TRANSACTIONS. " 499 



The matter he found best suited to the purpose is stiff clay, tempered in such 

 a manner as to be smooth and uniform, with the same reduced to powder, after 

 having been baked in an oven, as also after having, by a still stronger heat, 

 been converted into brick. The cistern is by turns filled with these different 

 materials, which are to be closely and uniformly pressed down, so as to leave the 

 surface quite level : in effecting which, great caution is required, more particu- 

 larly in regard to the powders; as they will not distinctly retain the impressions, 

 unless they have some small degree of moisture, or be very closely pressed down ; 

 in both which cases they acquire such a degree of cohesion, as of course must 

 render the experiments more or less imperfect. Things being thus prepared, in 

 order to try the necessary experiments, Dr.R. fixed the staples, and by their means 

 the branches, at the following heights, viz. 2 feet, 4 feet, and 8 feet; the result 

 of which experiments was as follows. 



When the brass balls, in weight to each other as 2 to 1 , were let fall on tem- 

 pered clay, from 4 feet to 8 feet respectively, the impressions made were on va- 

 rious trials found to be equal. — When the wood balls, being to each other as 2 

 to I, were let fall from the same heights, on dried clay pulverised, that from 4 

 feet generally made the deeper impression. — When the wood balls were let fall 

 from the same heights on brick-dust, that from 4 feet constantly made the deeper 

 impression. — When the lighter brass ball was let fall on tempered clay, from 2 

 feet and 8 feet, the impressions were to each other as 1 to 4. — When the lighter 

 wood ball was let fall on dried clay pulverised, from the same heights, the im- 

 pressions were, so far as the eye could judge, nearly in the proportion of 1 to 3. 

 — When the same ball was let fall on brick-dust, from the like heights, the 

 impressions were not much short of the proportion of 1 to 2. 



From these experiments it plainly appears: first, that the impressions made in 

 soft clay are in proportion to the heights, whence the balls are let fall, conse- 

 quently as the squares of their respective velocities. 2dly. That the impressions 

 in pulverised clay, recede considerably from that proportion, being as it were in 

 the medium between the squares of the velocities and the velocities themselves. 

 3dly. That the impressions in brick-dust are nearly in a subduplicate proportion 

 o the heights whence the balls are let fall, consequently vary but little from 

 the proportion of the velocities acquired. Whence Dr. R. apprehends it clearly 

 follows, that the impressions made in soft bodies, by hard ones striking on them, 

 vary from each other, according to the degree of cohesion in the respective soft 

 bodies; and that the impressions would be in exact proportion of the velocities, 

 could their form be perfectly retained by bodies quite void of cohesion. 



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