340; History of' Mechanical Inventions and 



tern, perhaps the greatest of all, is the method of stopping the 

 tube for carnage. This is done by screwing up a cushion against 

 the opeti end of the tube, which cannot fail to convey dust, air, 

 and moisture with it, and thus render the filling of the tube- 

 imperfect ; and the mercury not filling the cistern will be oxi-' 

 dated from the agitation it must receive in carriage. > 



I think it will be generally allowed, that the more perfect 

 any instrument can itself be made, the more will the cor- 

 rections and chances of error be reduced ; the results will be- 

 more easily obtained, and become much more deserving of con- 

 fidence. — I have the honour to be, 



very respectfully yours, 



John Adie. 



Edinburgh, 4th September 1829. 



Art. XXL— history OF MECANICAL INVENTIONS 

 AND OF PROCESSES AND MATERIALS USED IN 

 THE FINE AND USEFUL ARTS. 



1 . Mr Sevan's Experiments on the Moduhis of Torsion. 



The resistance of bodies to twisting has never yet been care- 

 fully examined, and practical men are under great obligations 

 to Mr Bevan for his very valuable experiments on this sub- 

 ject. 



In order to find the deflection b or quantity of twisting in 

 inches and decimals, Mr Bevan has given the following for- 



mula -^^TfT = ^> h being the length of a prismatic shaft strained 



by a given force w in pounds avoirdupois acting at right 

 angles to the axis of the prism, and by a leverage of given 

 length = r ; the side of the square shaft being = d, and T 

 being the modulus of torsion in the following table, Z, r, 6, and, 

 d being in inches and decimals. 



Table of the Modulus of Torsion. 



o -n Modulus of 

 species of Wood. »pecmc rpo^gioj,. Observations. 



V gravity, p^^^^^^ 



Acacia, - - .795 28293 Not quite dry. 



