452 Ignition of an explosive Gaseous Mixture. [Mar. 21, 



It was not possible to work with the pendulum chronograph (good 

 as this instrument is for other branches of research), as its length of 

 traverse is too limited, and the difficulty of subdividing tuning fork 

 traces is found to be very great, since the velocity of the pendulum 

 varies from zero up to a maximum during its swing ; this being so, a 

 new form of chronograph has been devised to meet as far as possible 

 all the requirements of the case, by means of the instrument. The 

 following results have been obtained : 



1. The 2oiro~oth second can be measured with ease, and periods of 

 time differing from T Vh second to -2 o^oo^h second can be recorded 

 on the same moving surface. 



2. The surface which receives the record moves at a velocity 

 which is practically constant during the traverse of 50 cm. ; also its 

 velocity can be varied between wide limits. 



3. A large number of time records can be made side by side, all 

 records being made in straight lines. 



4. Fractions of recorded vibrations of a fork can be subdivided by 

 means of a micrometer microscope. This is not the case with vibra- 

 tions recorded on a surface attached to a pendulum, where the velo- 

 city varies from zero up to a maximum at the middle of the swing. 



The electromagnetic styli, by means of which events are marked, 

 are so constructed that their period of " latency " is almost absolutely 

 constant, and their electromagnets are so wound that no sparking 

 takes place on breaking the circuit. 



A moving surface is carried on a carriage, which is propelled by 

 means of a falling weight, which after a certain velocity has been 

 attained is removed, the surface then moves with a velocity which is 

 found to be practically constant, for the limits between which a time 

 record is made. 



The chronograph is used in conjunction with a steel tube in which 

 the explosions take place. At even distances along the axis of the 

 tube, conducting bridges, eight to ten in number, of Dutch metal 

 insulated from the tube, are placed ; each bridge is connected electri- 

 cally with a recording stylus, so that as each bridge is broken by the 

 explosion, a mark is made on the surface of the chronograph ; 

 these markings when duly interpreted provide data for constructing 

 a curve, which indicates the rate at which the velocity of the 

 explosion is changing during its propagation. 



The rest of the paper treats of the methods by means of which the 

 errors due to the use of electromagnets in chronographic work have 

 been dealt with and reduced as far as possible. 



