Mr, Austin on a nezv Construction y C^c, 139 



given me hopes that it might be admitted to a place in your 

 laboratory. 



It was my wish, in tlie construction of this instrument, to 

 have made the piston of glass only, without any packing of 

 flax or thread for fitting it to the barrel ; but I have not ven- 

 tured to attempt so much. And perhaps as the packing is 

 only a very small ring of a known substance, and as that may 

 be varied at pleasure, by using for the purpose silk, or wool, 

 or leather, or elastic gum, the effect in any experiment may 

 be easily appreciated. The water which lubricates the barrel, 

 and with which the joints are wetted in the first instance only 

 (for the permanent junctures require no second wetting, but 

 remain perfect for any length of time) may require also to be 

 taken into the account, and if on occasion it should be thought 

 preferable, oil may be used in the place of water. Should it 

 however appear to you, that the want of a piston of glass, 

 which should act without the addition of any foreign substance, 

 is an important defect, you may possibly get one executed 

 perfectly by some of those ingenious and excellent artists 

 who are to be found in London. You will probably also im- 

 prove on the form and construction of this first instrument 

 (as far as I know) made of glass for the purposes mentioned. 

 I have myself partly executed another of more general ap- 

 plication on similar principles, of which, with your kind per- 

 mission, I shall give you an account, as soon as I have com- 

 pleted it. 



The great impediment to the forming of pneumatic instru- 

 ments of glass, appears to me to have arisen from the difficulty 

 of making the necessary joinings, in such a manner as to be 

 capable of sustaining very considerable pressure, to be easily 



MDCCCXIII. U 



