1896.] on Chronographs and their Application to Gun Ballistics. 181 



the Noble chronograpli, to grind tlie roughness out of the mechanism 

 by running it for some time. 



On a hinged frame of ebonite are placed a row of forty steel-pointed 

 pins, screwed into the ebonite so as to allow of accurate adjustment. 

 The frame is brought up to a fixed stop, and clamped by means of 

 two cam clutches. Each pin is carefully adjusted, to be at a uniformly 

 small distance of about -^^q- inch from the surface of the drum. The 

 ebonite frame is capable of traversing from right to left, so that each 

 point is opposite a different surface of the drum, for the convenience 

 of making a series of experiments without re-smoking the drum. 

 Each pin is connected by insulated wires with a binding screw on the 

 bed-plate. On the left edge of the drum is a carefully divided circle, 

 reading by means of a vernier to minutes of angle, and with care to 

 half this accuracy. 



Wires run from the secondary poles of a series of induction coils 

 to these binding screws. Thus 1 and 2 binding screws are connected 

 with No. 1 coil, 3 and 4 to No. 2 coil, &c. In this way I have two 

 records on the drum for each primary circuit. The primary circuits 

 of the coils are connected with plugs (which I shall presently describe) 

 screwed into the gun. 



Now we come to a very important part of the instrument, viz. the 

 means of timing the speed of the revolution of the drum. In my first 

 experiments, years ago, I employed the usual method so much in vogue 

 then and now, viz. tuning forks. A tuning fork, as you know, vibrates 

 so many times a second according to its note. Thus, for instance, the 

 middle C corresponds to 256 double vibrations in a second. To employ 

 these a small stylus is fixed to the tuning fork, which presses lightly 

 on the drum ; as the drum revolves a sinuous line is formed by 

 scratching off the smoked surface. I found, however, by careful trials 

 that you could not depend on these records, owing to different atmo- 

 spheric conditions and the varying surfaces of the drum. Nor does 

 this seem unreasonable when we look into the matter. In the first 

 place the vibrations of a fork are affected by temperature and baro- 

 metric pressure ; these are more or less known and could be allowed 

 for. We might also correct for the additional weight of the stylus, 

 but it seems to me more difficult, nay impossible, to say what the 

 vibrations are under the friction of the stylus on the surface of the 

 drum with varying thickness of carbon deposit. Moreover the 

 trouble of working out of the tuning fork records is considerable; 

 and with the circumferential speed necessary for recording millionths 

 of a second, forks with a very high note have to be employed A fork 

 giving the middle C, before mentioned, would be useless for this 

 purpose, but the higher the rate of vibration the greater would be the 

 retarding effect of the stylus recording the vibrations. 



The stop watch arrangement employed by Sir Andrew Noble is 

 not applicable to this instrument, nor is it, I think, a very accurate 

 method of timing. 



I have, after many failures, worked out a method which experience 



