TRANSACTIONS OF THE SECTIONS. 593 



Iir. MATHEMATICAL INSTRUMENTS AND 

 MECHANICAL ARTS. 



On a new Sympiesometer, By Professor Forbes, F.R.S. 



A BAROMETER acting by measuring the volume of a confined 

 portion of air, first recommended by Dr. Hooke, has been re- 

 cently constructed in a convenient form, under the name of the 

 Sympiesometer, by Mr. Adie of Edinburgh. The chief difficulty 

 iound m operating with this instrument consists in ascertaining 

 the precise temperature of the inclosed air. This is proposed 

 to be accomplished by placing both the gaseous ball and that 

 of the attached thermometer in one common chamber sur- 

 rounded with mercury ; whilst the difFerence of temperature 

 which may exist between that mercury and the external air is 

 determined by means of a small differential thermometer 

 1 he scale of the mercurial thermometer is read downwards, and 

 the volume of gas is indicated by a thermometric scale of its 

 expansions under a constant pressure of 30 inches. 



On the construction of Achromatic Object-G lasses. % David 

 Dick, Architect and Engineer, Edinburgh. 



Having several years ago attempted the construction of a 

 triple object-glass of 4 inches diameter, of which the interior 

 surfaces were cemented together, as recommended by the late 

 Professor Robison, Mr. Dick found that, when the surfaces 

 were found to coincide, it was rather difficult to separate them 

 without scratching, and therefore preferred to proportion the 

 radu of curvature so as to leave a small interval of the meniscus 

 form, which was filled up with the cementing substance. This 

 mode of construction suggested to him the possibility of em- 

 ploying a cementing substance having such an action on the 

 green light, in relation to that of the two sorts of glass, that 

 the colours of the secondary spectrum might be diminished if 

 not entirely removed. By referring to the discoveries of 

 feir JJavid Brewster regarding the action of the different re- 

 fractmg media on green light, it was found that Canada bal- 

 sam, oil of turpentine, and in a very high degree the oil of 

 cassia, were all possessed of the quahty sought for, and the 

 author has in fact succeeded in the construction of object- 

 glasses of considerable size, which produce images almost, or 



1834. 2 Q 



