Ronketti's sympiesometer, or air barometer, 1839.
© Science Museum / Science & Society Picture Library
Mercury barometers needed to have two tubes that were at least 33 inches long, and were generally cumbersome to carry around. To allow for readings of air presure by a pocket instrument, a much shorter glas tube was doubled back on itself to fit a small case, and the moving agent was a combination of air, spirit and mercury. This example was made by Carpenter and Westley to a design registered by Ronketti in 1839.