Mr. Bevan on Measuring the Force of Pressure. 285 



pressure, in small spaces, with considerable accuracy, I take 

 the opportunity of offering it to the public through your use- 

 ful publication. 



If we take a leaden bullet of any determinate diameter, and 

 expose it to pressure between plates of harder metal, made 

 to approach each other in a parallel position, the bullet will 

 be compressed or flattened on two opposite sides in an equal 

 degree ; provided the lead is pure, the degree of compression 

 will indicate the amount of pressure. With a graduated press 

 of the lever kind, it will be easy to form a scal6 of pressure 

 corresponding to the different degrees of compression, until 

 the ball is reduced to a flat circular plate, of about one-fifth 

 of an inch in thickness ; and it will be found that an ordinary 

 bullet of about five-eighths of an inch diameter will require a 

 pressure' of near 4000 pounds to effect this degree of flattening. 

 Suppose,'therefore, we wish to measure an actual pressure sup- 

 posed to be nearly twenty tons, we have only occasion to place 

 ten or twelve of these balls at a proper distance asunder, so as 

 not to be in contact when expanded, and then to measure by 

 good callipers, or other suitable means, the compression of 

 each ball, either by its thickness or diameter, and afterwards 

 add into one sum the particular pressure due to each ball, 

 from the scale first made by using the lever press before 

 mentioned. 



By this mode I have ascertained the amount of friction of 

 an iron screw press, with rectangular threads, to be from 

 three-fourths to four-fifths of the power applied ; or the actual 

 pressure has not exceeded four or five tons, when the calcu- 

 lated pressure, if there had been no friction, would have been 

 twenty tons. 



The larger the ball, the greater will be the pressure neces- 

 sary to reduce it to a given thickness. An ordinary leaden shot 

 of one-eighth of an inch diameter will require nearly 100 

 pounds to compress it to a flat plate. 



By using a ball of five-eighths of an inch diameter, I have 

 found the actual pressure of the common bench vice to be 

 about two tons ; when under the same force, if there had been 

 no friction, the pressure would have been eight tons. 



In the practical application of these balls, it will be conve- 

 nient to make a small impression upon them with a hammer, 

 before they are placed between the plates, to prevent them 

 from rolling out of their proper position; this operation will 

 not be found to interfere with the result, as it is the ultimate 

 compression only that is sought, and which is not aft'ected by 

 that of a smaller degree before impressed. This property 

 will also be found very convenient; for wc may use the same 



substance 



