834) Mr Forbes on the Defects of the Sympiesometer, 



Art. XXII. — On the Defects of the Sympiesometer, as applied 

 to the Measurement of Heights. By James D. Forbes, Esq. 

 Communicated by the Author. 



The Sympiesometer is an instrument more attractive at first, 

 perhaps, than in longer experience, and in its capabihty for ex- 

 pressing rather than receiving minute impressions. This will 

 account in some measure for the very partial adoption of a 

 contrivance, the excellency of the theory, and the ingenuity 

 of the inventor of which all will admit. The principle indeed 

 is by no means new, having been discussed by Hooke and 

 others in the earlier Philosophical Transactions ; but Mr Adie 

 has given it an elegance and an accuracy which its older pro- 

 jectors never aniicipated. , 



The portable sympiesometer, notwithstanding its lightness^ 

 is not likely often to replace the barometer in the hands either 

 of the practical man or the refined philosopher ; and briefly 

 for these reasons : It is as expensive and even more so than 

 a plain mountain barometer. Though not nearly so liable to 

 break as the mercurial tube by concussions, these will readily 

 separate the oil, and sometimes render the instrument equally 

 useless for the time, till it has undergone a rather hazardous ope- 

 ration. It must be kept in one position only, — a material objec- 

 tion, whereas a proper barometer maybe travelled in any posi- 

 tion. Though the space corresponding to an inch of mercury 

 is far greater, the viscidity of the oil is so considerable, that it 

 is impossible to read off* the height to less than t Jo of an inch, 

 and it is much to be doubted if the level of the oil be true to 

 that quantity. A barometer of similar expence will be read off" 

 with secure accuracy to one-fifth of that space. Any barome- 

 ter tolerably constructed will take its level in half a minute, but 

 the sympiesometer will not merely require for the same pur- 

 pose four or five minutes, as the inventor in his pamphlet states, 

 but I mean to show that the time thus required may be actUr- 

 ally indejinite, and to explain some weighty objections con- 

 nected with this, which, if my observations do not deceive me, 

 affect the instrument, so as to render it unfit in its present state 



