378 Mr. R. W. Wood on a 



angle 6 may be laid off. The modulus of the cell is best 

 changed by varying either A or B. This may be conveniently 

 done by making each of either the long pair or the short pair 

 of links like the legs of a pair of proportional dividers. 



A small model of an instrument on the above lines has been 

 constructed and found to work very easily and accurately. 

 Like all hyperbolagraphs, however, the range of motion is 

 limited (although larger in this than in most forms), and a 

 considerable amount of time is necessary for the preliminary 

 adjustment. For these reasons I have generally found one 

 of the preceding graphical solutions more rapid and con- 

 venient, especially when a number of curves are to be drawn 

 on the same sheet. 



This application of the quadruplane, which occurred to me 

 recently while making an application of the Peaucellier 

 linkage to a concave grating mounting *, seems so simple and 

 obvious that I feel sure it must have occurred to others as 

 well as myself; but as I have not been able to find any sug- 

 gestion to this effect in any of the papers on the subject that 

 I have examined, I have ventured to present the foregoing 

 description as another illustration of the practical application 

 of the beautiful geometrical discovery of Prof. Sylvester and 

 Mr. Kempe. 



Ryerson Physical Laboratory, 

 University of Chicago, U.S.A., 

 January 1896. 



XXXIX. A Duplex Mercurial Air-Pump. 

 By R. W. Wood f. 



IN working with highly exhausted tubes, such as are used 

 for the production of the Rontgen rays, one of the 

 difficulties met with is the speedy deterioration of the vacuum 

 due to the liberation of gas from the electrodes and the glass. 

 If the tube be thoroughly heated, while on the air-pump, this 

 trouble is partially remedied, but even with this precaution 

 the tubes are not very durable and have to be pumped out 

 frequently. In order to overcome this difficulty I have con- 

 structed a new form of mercurial air-pump, which can be 

 made on a very small scale and attached permanently to the 

 Rontgen tube. By this arrangement, any traces of gas that 



* " On the Use and Mounting of the Concave Grating as an Analyzing 

 or Direct Comparison Spectroscope," The Astro-Physical Journal/ vol. iii. 

 p. 47 (Jan. 1896). 



t Communicated bv the Author. 



