1 18 Description of the different Methods 
These plugs must have a longitudinal groove, that they 
may glide along the rod and suffer the water to escape. 
Observation . 
9th, Instead of a rod of hollow wood you may em- 
ploy, with advantage, either a tube of tin plate about 
four millimetres in diameter, terminating at the lower ex- 
tremity, which must be inserted in the cartridge in a trun- 
cated cone, and an orifice of two millimetres ; or a leaden 
pipe drawn in the manner of wire- drawers, having the 
same dimensions as the above, and whose resistance may 
be sufficient, if you take care to introduce into it, while, 
you drive in the wadding, a rod which may exactly fill 
the interior vacuity. 
If you have at hand any kind of composition capable 
of acquiring hardness in a little time * at the bottom of 
the water, you may substitute for the rod and metallic 
tubes a flexible tube of cloth done over with pitch or 
gum. In this case, it will be necessary to introduce the 
priming rod into the tube, while you drive in the wad- 
ding to prevent its depression. The cloth of the tube, 
the upper extremity of which is destined to rise above 
the hole in the rock, must be sufficiently thick and strong 
that the pressure of the water, which I suppose to be 
some centimetres above the ground, may not flatten it, 
even if the liquid should introduce itself between the 
tube and the composition. 
XII. Method of blowing up Mocks under Water at any 
Depth . 
10th, This method, on the first view, has a resem- 
blance to that first described, since a tube of tin plate is 
employed in it $ but it differs essentially from it in this 
* A mixture of quicklime and plaster newly calcined would, perhaps, fee of 
this kind. 
