1896.] on Chronographs and their Application to Gun Ballistics. 183 



covering the drum with paper is out of the question ; for the spark, 

 taking the line of least resistance, goes through the thinnest part of 

 the paper, which may or may not be directly opposite the points at 

 the moment the spark occurs. "When extreme accuracy is not 

 required, paper may conveniently be employed, as the paper, on being 

 removed and varnished on the back, may be kept as a record of the 

 experiment for future reference and measurement. 



The difficulty of obtaining a uniformly smoked surface giving a 

 minute centre for exact reading has been overcome by the following 

 simple means. A small lump of paraffin wax about the size of the 

 tip of one's little finger is dissolved in half a pint of benzole ; a rag 

 saturated with this solution is rubbed over the drum. The drum is 

 smoked with a large flat wick saturated with a moisture of equal parts 

 of paraffin oil and rape seed oil. The nature of the records obtained 

 can be varied at will, according to the amount of wax dissolved in 

 the benzole, but all have a distinctly defined minute centre, which 

 can be read to the greatest nicety. The method adopted of reading 

 the records, is to stretch a fine hair in the centre of a brass frame, 

 fitting with steady pins the supports of the drum centre. The hair 

 is so arranged as only just to clear the surface of the drum. A 

 magnifying glass enables one to bring the record marks exactly under 

 the stretched hair. 



We now come to the application of these instruments for measur- 

 ing gun ballistics. For external ballistics the usual screen is a 

 series of copper wires stretched across a wood frame. The cutting of 

 this wire breaks the circuit and gives the record. There is no doubt 

 that the cutting of a wire in this manner is not perfectly accurate, 

 and to a slight extent would vary with the size of the screen ; but 

 for ordinary work of getting the muzzle velocity of a shot, when 

 the screens are placed 120 feet or more apart, it is sufficiently good. 

 Bashforth employed a different form of screen, as he required the 

 circuit to be remade immediately the shot had passed through. In 

 this a spring, whose play was limited by a hole in a copper plate, was 

 held down to the lower surface of the hole by a weighted thread. 

 As the thread was cut the spring, rising to the top of the hole, broke 

 the circuit and remade it. In this method, as I have experimentally 

 proved, the breaking of the circuit is not very exact, but near enough 

 when the screens are far apart, I employed a somewhat similar 

 arrangement with my drop-weight chronograph, only using broad flat 

 springs to avoid the rubbing of the spring against the side of the hole, 

 which sometimes occurred in the Bashforth screens. 



For internal ballistics when we have to measure the passage of a 

 shot over plugs placed two inches apart, the utmost accuracy of break 

 is required. Sir A. Noble used a cutter plug which severed a wire 

 as the shot forced up an inclined plane. Unless the shot exactly fits 

 the bore, which of course with the ordinary projectile it does not, 

 considerable errors arise from the use of such cutter plugs, as we 

 never know the exact position of the shot when the wire is actually 



