222 



THE WORLD'S ADVANCE 



perature was raised to a point sufficient 

 to distill the mercury, mercury vapor 

 was formed, which passed through the 

 tubes and condensed in the other two 

 jugs. Pure gold remained, and the mer- 

 cury could be used repeatedly. 



The writer has witnessed the selling of 

 a mercury still of this type, by a miner 

 who had taken out all the gold he could 

 carry, to a newly arrived prospector for 

 the sum of $175. It would cost probably 

 no more than fifty cents to construct. 



To Study the Acoustics of the Air 



In the accompanying drawing is shown 

 an instrument which will give excellent 

 results in studying the acoustic proper- 

 ties of a room. 



It consists of a horn, A, which may be 

 an ordinary phonograph horn, for col- 

 lecting the air vibration. At the small 

 end of the horn a microphone, F and D, 

 should be attached. One side of the 

 microphone should be made of a mica 

 diaphragm to the center of which a small 

 carbon projec- 

 tion is fasten- 

 ed, and both 

 fastened to the 

 end of the 

 horn. The 

 other contact 

 of the micro- 

 phone should 

 b e supported 

 by a brass 

 frame, /. The 

 carbon should 

 be fastened to 

 the end of the 

 threaded rod, 

 /, which should 

 be made ad- 

 justable by the knob, E. The frame 

 is to be constructed of %-inch brass 

 rod. A wood box, B, should be built 

 about the microphone, and attached to 

 the horn by brass strips, H, H, and by 

 brass screws, K, K. The remainder of 

 the equipment consists of a set of two 

 dry cells, G, and a pair of sensitive 

 double-telephone receivers. 



Sound waves will be collected by the 

 horn and registered in the receivers, 

 after the microphone has been adjusted 

 properly. Records of sound waves as 



affected by temperature and air pressure 

 may be kept. 

 Contributed by 



FRANCIS B. DEARDORF. 



Carrying Case for Small Drills and 

 Taps 



A handy carrying case for small taps 

 and drills can be made from a short 

 length of brass tubing. The tube should 



be sawed into 

 two lengths, 

 one of which is 

 to serve as a 

 cap for the 

 case. One end 

 of each tube 

 should be 

 closed with a 

 brass disc 

 which is first 

 soldered in 

 place and then 

 filed to the size 

 of the tube. A 

 strip of brass 

 an inch wide 

 should be 



rolled, fitted into the neck of the longer 

 tube so that half of it protrudes, and 

 soldered. The shorter end the cap 

 fits on over this. 

 Contributed by 



EARL B. WILLIAMS. 



Odd Use for Old Files 



The old custom in machine shops of 

 tacking a strip of sandpaper under each 

 gas jet for the purpose of striking 

 matches can be improved upon consider- 



