38 Prof. Forbes on the Application ofjhe 



It may be proper to remark, that Professor Oersted's instru- 

 ment for indicating the compressibility, of water, consists merely 

 of a very sensible thermometer, constructed of water, and having 

 the end of the tube left open. The tube being capillary, a short 

 column of mercury rests on the surface of the water, indicating 

 its volume at any moment ; and the whole is immersed in water 

 contained in a strong vessel, to which pressure is any how com- 

 municated, so that the thermometer-shaped vessel of glass being 

 equally pressed within and without (the neck being open), is 

 unaffected by pressure, and the true change is perceived in the 

 volume of water which it contains. 



The applications of this form of instrument are very nume- 

 rous ; we may take as examples, 



1. The determination of the tension of gas or air in a com- 

 pressed magazine, as I have just suggested. 



2. The measure of elasticity of high-pressure steam. 



3. The determination of the degree of compression under 

 which bodies change their state, when such experiments can be 

 performed in glass vessels, as in the case of the condensation of 

 the gases into liquids, the pressures as stated by different au- 

 thors varying extremely, and being confessedly imperfect ap- 

 proximations. 



4. The ready determination, by inspection, of the pressure 

 per square inch exerted by Bramah's press at any instant. 



Nothing could be easier than to convert the instrument as 

 above described into a self-registering one, by simply inserting 

 an index of glass, which may be drawn back by the little mer- 

 curial column, just as in Six's thermometer. We should thus 

 be enabled to determine the operation of causes by their nature 

 concealed from direct view ; as, 



5. The force exerted by water in the act of freezing, in a 

 manner much more direct and satisfactory than that of the Flo- 

 rentine Academicians, because it would not be necessary to cause 

 the recipient to burst, the maximum expansive force being indi- 

 cated by the register. 



6. The force of fired gunpowder ; and even of dead pressure 

 and of percussion in a variety of cases. 



7. The depth of the ocean by the measure of the pressing 



