MEASUREMENT OF PRESSURES 207 



which must be properly done if trustworthy results are to be obtained 

 efficiently. The details of such experimental layouts will of course de- 

 pend very much on the specific problem and the available facilities, and 

 as a result there are not many principles and procedures sufficiently 

 general in application to merit a detailed discussion here. There are, 

 however, several basic considerations which must always be taken into 

 account and are discussed here, together with brief descriptions of ex- 

 perimental arrangements which have been used for some of the large 

 and small charge work at Woods Hole. 



The most obvious characteristics peculiar to underwater explosion 

 work are the facts that high explosives by definition react violently and 

 can cause serious damage, wanted or otherwise, in their vicinity; that a 

 sufficiently large body of water must be available; and that an explosion 

 happens only once. If work with small charges only is intended, it is 

 not too difficult to provide a suitable volume of water, necessary safety 

 features are readily obtained, and arrangements of gear can be made 

 with small expense in manpower and materials. For example, tests 

 using standard detonator caps and/or charges up to an ounce or so in 

 weight (usually tetryl) have been performed at a number of laboratories 

 in tanks of steel or concrete of moderate size, say three feet or more in 

 each linear dimension, and the major precautions are those of proper 

 explosives handling and prevention of interference with other activities 

 in the immediate vicinity. 



Charges up to a pound or more in weight can usually be fired in 

 small ponds or off docks not too near buildings. At Woods Hole, for 

 example, a great number of investigations using charges as large as 

 one-half pound have been performed off a dock in twenty feet of water, 

 an arrangement which was particularly convenient because booms and 

 tackle at the edge of the dock could be used for easy handling of gear 

 and recording equipment could conveniently be housed nearby. Arti- 

 ficial or natural ponds can be used which still permit shore installation 

 of handling and recording equipment. At Woods Hole, a shallow 

 natural pond was enlarged by excavating to provide water eight to ten 

 feet deep in which charges up to five pounds were fired. Larger charges 

 were impractical because of the small depth and short time interval be- 

 fore reflections occur from the boundaries, and because of filling up by 

 loosening of the sides of the pond. 



Damage to the walls and limited volume of water will probably be 

 the limiting factors on the size of charge which can be fired in a small 

 body of water, and work with larger charges, such as service weapons, 

 has without exception been done either in lakes or coastal waters of the 

 ocean. Many types of gear have been used for different purposes in 

 the small charge work, but the most useful arrangement for many pur- 

 poses has been to support charges and gauges or other equipment on a 



