412 Dr. J. Shields on a Mechanical Device for 



order to take a reading it is first gently tapped, the lower 

 meniscus is then set exactly at the zero line by means of the 

 screw at the bottom, and the temperature correction as 

 indicated by the correcting instrument at once read off lest 

 the heat of the body should cause any alteration ; the height 

 of the barometer is then observed at the top of the instru- 

 ment, which, after subtraction of the temperature correction, 

 gives the barometric height reduced to 0° C. 



By considering the correcting instrument it is obvious that, 

 the temperature remaining constant, the rise or fall of the 

 barometer is accompanied, after setting the lower mercury 

 meniscus to zero again, by an upward or downward displace- 

 ment of the horizontal thread of mercury, and, consequently, 

 to an increase or decrease in the correction. Similarly, a 

 rise or fall of temperature is accompanied by an increase or 

 decrease in the correction. 



The instrument I have just described is one out of four 

 possible modifications. Both scales may be fixed whilst the 

 barometer and thermometer are displaced simultaneously or 

 vice versa. Again, the barometer and correcting scale may 

 be fixed whilst the other parts are adjustable or vice versa. 



The first two will probably be found most useful. Distinct 

 advantages might be gained by fixing the barometer with the 

 thermometer above it, and at the same time etching the 

 barometer and correction scales on the same piece of plate- 

 glass mirror, which could be placed behind them and be 

 moved vertically by a set screw at the bottom of the 

 instrument. 



University College, London. 



Appendix. 



Fig. 3 illustrates a convenient graphic method for obtaining 

 the temperature correction of a barometer or other column of 

 mercury. 



The ordinates represent the height of the mercury column * 

 and the abscissae the correction in millimetres. The correction 

 for any temperature not represented by a diagonal line 

 passing through the origin is easily obtained by a graphic 

 interpolation. As fig. 3 is too much reduced in size to be of 

 any value, a diagram of convenient size may be obtained by 

 plotting the ordinates on a piece of curve paper ruled into 



* In this particular case the heig-ht is supposed to be measured with a 

 glass scale. If a brass scale is used the correction is always about 

 GO per cent. less. 



