504 Prof. R. Threlfall and Mr. J. F. Adair. On the 



work in the lathe. One end of the cardboard cylinder thus obtained 

 was closed by having a wooden plug glued into it. Through this 

 plug a hole was drilled to fit the detonating caps we were able to buy. 

 A cap was pushed, open end first, into the hole till it was flush with 

 the inner surface of the plug ; it was secured in this position with 

 the help of shellac varnish. The empty portion of the cap was then 

 filled with a mixture of fulminate of mercury and guncotton fluff, and 

 this filling was allowed to cover the bottom of the cartridge case 

 above the plug. Finally, some bees'-wax, resin, and linseed-oil luting 

 was poured over the surface of the upper portion of one of the plaster 

 cylinders, and this was then pushed home, so that the priming rested 

 on the loose mixture above the plug. 



Finally, the whole thing was dipped into some of the same luting, 

 so as to give it a waterproof coat. The detonators were then tested 

 with a telephone and stored. 



The advantage of this method is that as each operation is quite 

 simple the detonators can be formed by unskilled labour; a con- 

 sideration, if many have to be made. The luting must have some 

 linseed oil with it, otherwise it is apt to crack in cold weather. We 

 have never known a detonator so constructed to miss fire, though 

 missfires often occurred till we used the fulminate and guncotton 

 priming. The collodion paste is also very suitable, as it protects the 

 delicate bridge. In most cases the charges to be fired were contained 

 in tin cases, big enough to hold one 9-oz. disk of guncotton. The lid 

 of each tin had a cylindrical tin tube projecting from it to a distance 

 of 4 inches; this tube was not central, because 9-oz. disks are 

 bored with two holes. The detonators were secured in these tubes 

 simply by dipping the tubes in hot luting and then pushing the 

 detonators down till the cap projected to a sufficient distance. 



The charge was then placed in the tin, the detonator passing into 

 one of the holes of the cotton, and then the junction between the lid 

 and the tin was made secure with luting. For smaller charges than 

 9 oz. other receptacles were provided ; in most cases simply wrapping 

 the charge in mackintosh cloth was sufficient to keep out the water 

 long enough for our purpose, the joints being well secured with a 

 solution of india-rubber. One terminal of the detonators was always 

 put to earth. 



Apparatus for the Measurement of the Plates. 



As far as we can learn, it has hitherto been usual in the interpreta- 

 tion of the markings on a smoked plate simply to count the whole 

 number of fork-beats, and to estimate the fractions at best by means 

 of a pair of spring dividers. In our case this method is obviously 

 not on a par, as far as accuracy goes, with the other measurements. 



