TRANSACTIONS OF THE SECTIONS. 63 



cylinder at equal distances, in such a manner that the nut can be re- 

 volved freely, but cannot be otherwise displaced. 



From the entering part of the nut a projecting portion forms a 

 shoulder, which is graduated into equal parts. (This projecting por- 

 tion may be of any diameter greater than that of the cylinder.) 



On the exterior of the cylinder an index is fixed, by means of which 

 any number of revolutions of the nut, or any number of equal parts 

 of a revolution, can be ascertained. 



To prepare this instrument for use, the piston is to be retracted to 

 its lowest position, and the cylinder is to be filled with mercury (with- 

 out air bubbles) by pouring a sufficient quantity of this metal into the 

 glass that is attached to the stop-cock, and turning the plug for its ad- 

 mission within the cylinder. 



If, when the cylinder is fuU, and while some mercury still remains 

 within the glass measure, we turn back the plug of the cock, we 

 get the air-way of the plug filled with mercury ; and by pouring off 

 the superfluity, we have the instrument in a proper state to commence 

 graduating any tube for laboratory purposes. Thus, if the tube to be 

 graduated be about one third of an inch in diameter, if we open the 

 communication between the cylinder and the measure and propel the 

 piston by one whole turn of the nut, and then close the communication 

 between the cylinder and the measure by turning the plug of the cock, 

 we have within the measure a quantity of mercury, which, when poured 

 into the tube to be graduated, will give a tolerably long space for the 

 first division ; and such similar spaces may be successively marked by 

 repeating the process, until the whole tube be equally divided from 

 end to end. 



Notice of an Apparatus for determining the quantity of Carbonic Acid 

 Gas in deteriorated atmospheres, ^y Charles Thornton Co athupe. 



The apparatus consisted of a glass tube of about 24i inches in length, 

 and having an internal diameter of about half an inch. It was terminated 

 at each extremity by a brass cap, into each of which a brass stop-cock 

 was firmly screwed. The glass tube was divided into 175 equal parts, 

 by equal measures of mercury; and these divisions were numbered 

 upon opposite sides of the tube in such a manner, that let either end be 

 uppermost, the graduations might be instantly read. Every experiment 

 could thus be tested by a double reading, by simply inverting the tube, 

 and waiting a few seconds until the liquid employed for any examina- 

 tion had drained to its ultimate level. 



The liquid reagent to be employed was stated to be either a clear 

 saturated solution of quick lime in distilled water, or any aqueous solu- 

 tion of potassa, soda, or baryta. 



The mode of using the tube for ordinary purposes was described as 

 follows : 



Fill the tube with the air to be examined, by any of the well-known 

 means, and close the stop-cocks. 



Pour a drop or two of the reagent that may be preferred into the 



