104 20 
PART II. 
METHODS. 
(17) Gauges for measuring the Time-integral of the Pressure. 
When the development of new gauges was undertaken, it was recognised that the 
ideal to be aimed at was an apparatus that would give the complete time-history of the 
pressure at a given point in the water, but it was diilicult to see any way of arriving 
at this result. Hopkinson’s pressure-bar method (Section 22) did not seem suitable, 
especially under sea conditions, for measuring pressures which it was estimated would 
endure for several thousandths of a second, while on the other hand this period was 
too short to give much hope of success with any form of mechanical chronograph. 
First attempts were therefore limited to the more moderate aim of producing gauges 
that would measure (1) the maximum intensity of the pressure, and (2) the time- 
integral of the pressure, or in other words the duration of the pressure multiplied by 
its average intensity. 
After several failures, which need not be described, a successful time-integral 
gauge was produced in the form ehown in Fig. 34 (Type G). This gauge was designed 
with a view to the greatest possible mechanical simplicity and absence of friction. It 
is based on the principle that the momentum acquired by a body is equal to the time- 
integral of the force that has acted on it. ‘The working part is an easy-fitting steel 
piston, which rests on the rim of a small washer, with its bottom end exposed to the 
water. The top end of the piston carries a smal] copper cylinder, sprung into a thin 
corrugated metal jacket which keeps it centred. The central hole in which the piston 
works communicates with four others which act as an air reservoir. The air chamber 
is necessary both to prevent the piston from being forced up by hydrostatic pressure 
and to reduce the cushioning effect when the piston is driven in (Section 29). 
The pressure set up by an explosion acts on the bottom end of the piston and 
shoots it inwards, and after travelling a distance of 2 inches the piston hammers the 
copper against the eyebolt of the gauge, which acts as an anvil. The energy of the 
piston is absorbed by the copper, which is shortened to an extent determined by 
micronieter measurements. By aid of calibration experiments the momentum of the 
