302 Mr Haldane Gee and Dr. A. Harden on 



The vessel A, the stem of which is graduated and 

 calibrated, so that both the distance, and also the internal 

 volume of the tube is known between any two graduations, 

 is immersed in mercury up to a fixed mark (C) on the stem. 

 A ground-glass plate is then applied so as to close the 

 vessel air-tight. The apparatus is now raised to a con- 

 venient extent, and the level of the mercury inside at D, 

 and outside at E of the stem noted. 



The body of unknown volume is then placed in A and 

 the experiment repeated. 



I. To find the volume V of the airjn the vessel'A. 



Let the atmospheric pressure = ;;/ 



The pressure after lifting, i.e., m- DE — ?t 



The increase of volume CD due to diminution 



of pressure = v 



Then 



V + z; = 



n 



and 



V= ''^ 



7n — n 



II. Let x= the volume of the substance, 



Then in the second experiment N-x must^^be 

 written for V, its value calculated and the value of 

 X found by subtraction. 



A very similar apparatus (Fig. 2) was described by Leslie 

 in 1826 (^Quarterly Journal of Science and Arts, XXI, 374), 

 and was used by him to ascertain the specific gravity of 

 various powders, such as charcoal, flour, volcanic ashes, etc. 



To obviate the error arising from absorption of air he 

 determined the volume with different degrees of dilatation. 



W. H. Miller {Phil. Mag., 1834, V, 203), in 1834, intro- 

 duced a considerable improvement in the construction of 

 Say's instrument, which will readily be understood from the 



