"640 Mr Forbes wi tJie Defects of the Syvipiesometer^ 



It is clear from our explanation of the irregularities of the 

 sympiesometer that the cause (viz. any change of tempera- 

 ture) must be indicated by the thermometer after the irregu- 

 larity has been observed, since the explanation is founded on 

 the inaptitude of the thermometer to receive speedily slight 

 impressions. In the two former tables, where the tempera- 

 ture uniformly declined, the sympiesometer uniformly rose ; 

 and here, where the thermometer undergoing irregular changes, 

 and being nearly as high at the end as at first, the sympieso- 

 meter exhibits similar irregular movements. A little attention 

 to Table 3 will show that the changes incident to the indicated 

 height are succeeded shortly by such changes in the thermome- 

 ter, as, according to our hypothesis, form an explanation of them. 

 Thus the rise of .02 inches at Obs. 32 and 33, is succeeded 

 by a fall of 0°.2 at Obs. 34, and the fall of the oil at Obs. 34 

 had been caused by an evanescent current of air, producing 

 at Obs. 35 a minute rise of the thermometer. The continuous 

 rise of the oil in Obs. 35 to 37 is satisfactorily accounted for 

 by the considerable fall of the temperature from 66.4 to 65.2 in 

 the same observations ; nor can we doubt that the thermome- 

 tric rise of P.3 in the succeeding observations was accidental 

 from the approach of the body of the observer to the thermo- 

 meter only, since it descends immediately after, and since the 

 problematical rise of the instrument is readily explained by the 

 partial heating of the thermometer. Had it not been for this 

 obviously accidental heating, the oil, instead of declining, 

 would in all probability have risen steadily, till Obs. 39, ac- 

 companied with the descending temperature, indicated by 

 the thermometer in Obs. 39 to 41, where a rise is again ex- 

 hibited which had already affected the oil at Obs. 40. The rise 

 of the oil in the two last observations is to be imputed to an- 

 other descent of temperature not indicated when the obser- 

 vation ceased. 



