the Temperature Corrections of Barometers. 407 



must be subtracted * from the observed height to reduce it to 

 0°0. 



Graphic methods are sometimes used to obtain the tem- 

 perature correction, and one of the best of these, which was 

 first brought to my notice by Prof. Ramsay, is described in 

 the Appendix as it seems to be little known. 



During the course of an investigation it was necessary for 

 me to read and correct the barometer several times daily, 

 and as this operation became rather tedious I was induced to 

 make a barometer which indicated the height and the cor- 

 rection simultaneously. The construction of the barometer 

 presents no great difficulties, and as it is extremely useful in 

 its new form I now beg to lay a description of it before the 

 scientific world. It can be read with certainty to 0*1 millim., 

 which is sufficiently accurate for most purposes. Whether an 

 improved method of reading and better workmanship than I 

 have been able to bestow upon it would make it suitable for 

 meteorological observations must be left for meteorologists to 

 decide. For ordinary laboratory work, however, it meets all 

 the requirements. It is not necessary to know the temperature 

 at all ; and by mentally subtracting the correction as indicated 

 by the correcting instrument from the observed height (also 

 obtained directly by setting the barometer), the observer is 

 enabled in one entry to write the corrected height of the 

 barometer in his note-book. 



The most suitable form of barometer to employ with the 

 correcting instrument is that described by Dr. J. Norman 

 Collie f, but an ordinary syphon barometer may also be 

 adapted for the same purpose. The lower end of the baro- 

 meter is cemented or otherwise securely fitted into a brass 

 cap A, fig. 1, to which is attached a rod B, which moves 

 vertically in a guide in order to prevent the barometer from 

 rotating when it is raised or lowered by means of the screw G. 

 The barometer itself is kept in a vertical position by the 

 guides D D which are attached to the framework. The back 

 of the framework consists of a long, narrow board, the lower 

 end of which is shown at E, and to which the nut carrying 

 the screw is fixed. A plate-glass mirror F, carrying the 

 graduations, is firmly screwed down on the main frame by 

 means of the picture-frame moulding Gr Gr, which is planed 

 down at the back to such an extent that the mirror is held 

 tightly clamped in position. The plate-glass mirror is care- 

 fully graduated between 700 and 800 millims., and has also 



* For temperatures below 0° C. the correction must be added. 

 + Trans. Chem. Soc. (1895), p. 128. 



