of blowing up Rocks under Water . 115 
4, 5, or 6 metres : it is that employed at Carls crona. It 
is very ingenious, and seems hitherto to have befh un- 
known to the French miners. 
3d, But, before I begin to describe these methods, it 
may be of some use to mention here an interesting me- 
moir, printed in the Journal de Physique for the year 
1779, on the construction of air-boats proper for facili- 
tating the execution of all sorts of works under water, 
without employing pumping. C. Coulomb, the author of 
this memoir, after describing the method of constructing 
the air-boat, shews in what manner it is to be used. He 
points out the means by which it may be made to sink at 
pleasure, of placing the workmen under the box, of con- 
tinually renewing the air, of removing the rubbish and 
laying a foundation of mason-work at the bottom of deep 
water. He then calculates the time necessary for re- 
moving a metre in height from the bank of Quille-bceuf, 
which interrupts the navigation of the Seine ; and fore- 
seeing the cases in which mattocks or pick-axes would 
be insufficient for clearing obstructions from the bottom 
of the water, and where the hardness of the rock might 
require the use of gun-powder, he proposes two methods 
of blowing up rocks under water. 
In one, the workman placed under the box bores the 
rock, and introduces into the bottom of the hole a box of 
tin plate tilled with gun-powder, to which is soldered a 
small tube, also of tin plate, which rises above the water 
at ebb-tides, and which is stopped with some greasy mat- 
ter, after having been filled with a very weak composi- 
tion to serve as a train. The sea, as it rises, makes the 
air-boat float ; and when its lower edge has risen higher 
than the extremity of the tube, it is then removed, and 
when the ebb-tide uncovers that extremity, a person goes 
in a boat and sets fire to it. 
In the other method, which the author proposes for 
