a 



408 Dr. J. Shields on a Mechanical Device for 



a zero line etched on it, but the space 

 between zero and 700 millims. may be 

 left ungraduated. 



This particular form of mounting the 

 barometer, independent of the correcting 

 instrument which has yet to be described, 

 is in itself very useful ; as by setting the 

 lower meniscus of the barometer at the zero 

 line by means of the screw at the bottom ^ 

 of the frame, the uncorrected height can 

 be read off directly, and this obviates the 

 necessity of taking down the upper and 

 lower readings nnd adding or subtracting 

 as the case may be. 



Before proceeding to show how the £ 

 device for indicating the amount of the 

 temperature correction can be attached to 

 a barometer mounted in this way, it is well 

 to note that the areas of the upper and 

 lower reservoirs of the barometer are 

 supposed to be equal, and are in fact 

 approximately so, if these reservoirs are 

 cut from adjacent parts of the same piece 

 of tubing. Assuming now that the baro- 

 meter is accurately set, and that the 

 pressure of the atmosphere then changes, 

 if the pressure rises or falls n millim., 

 then, on again adjusting or setting the 

 barometer, any point on the stem will ob- 

 viously be raised or lowered n/2 millim. 

 Should, however, the cross section of the 

 capillary tube, which connects the main 

 stem of the barometer with the lower 

 cistern, be large when compared with the 

 cross section of the lower cistern, then the 

 above relation will not hold good. 



This source of error may be eliminated 

 either by making the cross section of the 

 capillary small, or by selecting a lower 

 cistern with a proportionally larger area. 

 It may also be eliminated, if necessary, 

 in graduating the scale of the correcting 

 instrument, but any slight error intro- 

 duced in this way has scarcely any ap- 

 preciable effect on the accuracy of the 

 readings. 



