Figure 37. Bucknill-Casella manometric sounder, invented by Lieutenant John T.
Bucknill of the Royal Engineers of the Royal Navy in 1870 to mitigate problems
with existing sounding systems. This sounder was based on Bourdon's tube, whose
curvature varied as a function of the pressure difference between the interior
and the fluid in which it was immersed. Louis P. Casella made the final product.