THK LAW OF BURNING OF MODIFIED CORDITK. 249 



exceptional lot was used, ami it will have to be referred to a special pressure-density 

 curve. 



2. Investigation of the Time Rise of Pressure (Cord Form). M.D. cordite has 

 heen made in various forms, some of which are only experimental. As with Mark I 

 cordite, the first form was cords ; since that time tubes, strips, and double tubular 

 forms have l>een made and experimented with. 



The first time rises I investigated were, then, of M.D. cordite in the cord form. A 

 time rise of such a cord, measuring 0'1265 inch in diameter, fired at 80 F. at a 

 density of 0'2448, is shown on fig. 4. The close agreement of this beautiful curve 

 with the points actually measured by the apparatus used in the experiment is an 

 indication of the accuracy of the arrangements. Having obtained this time rise, the 

 next step was the investigation of the law of combustion by parallel surfaces. 



The method employed was the following: Intervals of O'OOl second were worked 

 to. From the curve the pressure at O'OOl -0'002, &c. second was obtained. This 

 pressure corresponds to a certain density of gas obtained from Table A. Now this 

 gas is produced by a small reduction dr in the radius r of the cord, in other words, a 

 skin or lamina is burnt off and converted into gas. The available capacity of the 

 vessel for this quantity of gas is the total capacity less the volume occupied by the 

 unburnt cordite. One has therefore only to solve for dr under these known 

 conditions. This reduction dr then takes place in O'OOl second under an average 

 pressure which is obtained from the time rise. The average pressures 1 have taken 

 are those at half time in the interval. For instance, the average pressure during the 

 first O'OOl second is the actual pressure shown on the curve at 0'0005 second. In 

 actual practice, instead of working on the reduction of radius I have worked on the 

 reduction of diameter. 



Fig. 5 shows the results of this calculation for fig. 4 plotted in terms of reduction 

 in diameter and pressure. This figure also shows the lines I have selected to 

 represent ^the relation at temperatures of 60 F. and 80 F. 



It appears quite clear that the relation is expressed by a straight line, and that 

 therefore the power n is unity. The equation to the lines is of the form S = aP + C, 

 a varying with the temperature of the cordite. 



It is the existence of this constant C which has not been suspected l>efore, and 

 which, I think, shows the danger of assuming an equation of a theoretically perfect 

 form and then trying to deduce constants by trial and error. 



The meaning of the constant C can only be that below about O'l ton pressure the 

 law of reduction in diameter does not hold. Obviously, when P = 0*, S cannot 

 equal C. 



The cause of this change of law is, I suggest, that until some definite pressure is 

 attained in the vessel true explosion does not commence. I advance the following 

 explanation : When the charge is first ignited, only the cordite in immediate contact 

 with the igniter commences to burn, Cordite being a bad conductor of heat, this 



VOL. ccvu. A. 2 K 



