PROFESSOR H. B. DIXON ON THE RATE OP EXPLOSION IN GASES. 105 
of the tube; about four feet from the firing point, the flame broke a piece of silver 
foil stretched across the tube, the moment of rupture being marked on a moving plate 
by an electro-magnetic style. On reaching the further end of the tube, the flame broke 
a second strip of silver, the moment of rupture being recorded by a second style. To 
eliminate errors due to retardation of the electro-magnets, a blank exjaeriment was 
made immediately before firing the gases. The plate of the chronograph passed over 
two breaks, which could be adjusted so as to be broken by it at the same instant. 
In the preliminary experiment, the circuits of the two magnets were completed 
through these two breaks, and were thus broken simultaneously, the two styles 
registering their marks on the moving plate. The magnets, without their position 
being altered, were then connected with the silver bridges across the explosion tube, 
and the gases were fired by the moving plate striking one or other of the two breaks. 
The flame travelling down the tube broke the two silver bridges in turn, releasing 
the two styles, which again registered their marks on the moving plate. The two 
marks made by the first style gave the interval of time between the spark and the 
breaking of the first bridge, independently of the error of the electro-magnet; for 
both marks were equally affected by any error of retardation. In the same way, the 
two marks made by the second style gave the interval of time between the spark and 
the breaking of the second bridge, independently of the error of the electro-magnet. 
The difference between the intervals was the time taken by the flame to travel 
between the two bridges. 
This method of cutting out the errors of the chronograph is only valid when the 
current flowing through the electro-magnets is constant. A small resistance coil was 
interposed in the preliminary experiment in each chronograph circuit, to match the 
resistance of the silver-bridge and connections used in the actual determination. The 
current was supplied by a storage cell (Elwell and Parker) to each electro-magnet, 
and the time between the preliminary and the final experiment was never more than 
one minute. In all the later portion of the work a confirmatory experiment was 
made after the gases were fired, the connections being made as in the preliminary. 
In most cases the styles passed absolutely over the preliminary marks—merely 
deepening the lines, thus showing that nothing had been displaced. In some cases 
the preliminary and confirmatory marks were just visibly separated ; in these cases 
the mean position of the two was used to calculate the rate. In the few instances in 
which the confirmatory did not closely agree with the preliminary mai'ks the experi¬ 
ment was rejected. In order further to eliminate errors of the instrument a second 
experiment was always made as soon as possible after the first, the connections of 
the chronograph being all reversed—the first style being connected with the second 
bridge, and vice versd. The mean result of the two experiments is taken as one 
determination. 
The chief differences between my apparatus and that employed by Berthelot were 
the following :— 
MDCCCXCITI.—A. P 
