Mr. Samuel Seaward on High-pressure Gauges. 191 



It gives me pain to observe this display of ignorance ; for I 

 am sure I felt disposed to give credit to that gentleman for a 

 greater share of mechanical skill than what such doubt would 

 seem to argue. The making of such a joint is an every-day ope- 

 ration. However, to tranquillize his fears upon this point, I beg 

 to say, that if a trifling escape of air should take place at this 

 joint (which is quite absurd to suppose), it would not neces- 

 sarily prove fatal to the instrument; because the defect can in 

 a few minutes be detected and remedied by a method which 

 for simplicity and ease will, I flatter myself, almost satisfy the 

 fastidious notions of the worthy gentleman himself. 



The method of rectifying " my own " gauge, I ought to 

 have included in the description which I had the pleasure of- 

 transmitting to you; but it was inadvertently omitted: and I 

 beg to return my grateful acknowledgements to Mr. H. Rus- 

 sell for affording me the present opportunity of supplying the 

 omission. 



All gauges acting by the pressure of confined air require 

 occasional rectification, because, from the oxidation which 

 takes place on the surface of the mercury, a portion of the air 

 becomes absorbed ; and if oil or other liquid be employed in- 

 stead of mercury, there will be as great inconveniencies. 



It is proposed that the chamber A {vide description of the 

 gauge in your Magazine for January last) should be furnished 

 with a small screw plug, and the induction pipe (e) with a 

 small stop valve, similar to what is employed in the common 

 gauge: by the closing of the latter the pressure of the gas will 

 be shut off, and a communication made between the chamber 

 B and the external air : in a similar manner, if the screw plug 

 be removed, a communication will also be made between the 

 chamber A and the external air : therefore, if there should 

 be, from any cause whatever, a deficiency in the proper quan- 

 tity of inclosed air, it will thus be immediately supplied, and 

 the equilibrium restored. And with regard to any insensible 

 escape of air at this joint, it is proper to observe that as the 

 pressure of the gas is generally above 20 atmospheres, the 

 mercury will therefore be some height up the glass tube; con- 

 sequently the joint in question will mostly be sealed, and all 

 escape totally prevented. 



With many apologies for troubling you with such a long 

 letter, I remain, gentlemen, 



Yours &c. 

 London, March 8, 1824. Samuel Seaward. 



XXXIV. Me- 



