354 Mr. J. D. Chorlton. Examination of some of the [Mar. 19, 



So that the average difference for 1 centigram increase of weight 

 is 1*2 scale divisions, and as each division can be read to tenths, an 

 article can be weighed by the balance to 1 milligram. 



Joule says that he could weigh with this balance to the hundredth 

 part of a grain, that is to the fifteen hundredth part of a gram. 



To test this I made a number of determinations of the weight of a 

 glass cube which had been previously weighed by an Oertling balance 

 aud ascertained to be 207*020 grams in weight. 



The mean of these determinations gave a value of 207*0197 grams, 

 agreeing with the value obtained by the Oertling balance to less 

 than half a milligram. Some of the values, however, differed from 

 the mean by as much as 2 milligrams. Still, this is a remarkable 

 result for a balance made entirely of wood. 



Mercurial Air-exhauster (' Collected Papers,' vol. 1, pp. 623 — 627.) 



An account of this air-pump was read before the Manchester 

 Literary and Philosophical Society (vol. 12, p. 57 ; vol. 13, p. 58 ; 

 vol. 14, p. 12). 



The pump has been altered and improved several times, as may be 

 seen from Joule's papers ; the form in which I found it at Sale is, I 

 think, the final form it took after all improvements had been made. 



It consists of a large, thistle-shaped, glass vessel, a and &, one end 

 of which is connected by an india-rubber tubing to a glass globe con- 

 taining mercury ; the entrance tube, £, is blown out at c, and the bulb 

 so formed is ground into the neck of the thistle glass, so as to make 

 a joint which is impervious to mercury. To collect the pumped gases, 

 Joule finally used an inverted glass vessel, /, fitted with an india- 

 rubber cork, through -which passes the entrance tube, t, and the exit 

 tube, e. The entrance tube should be connected at h to the vessel to 

 be exhausted, and at g to a mercury manometer, which I was unable 

 to find. The glass work is supported by a wooden post, 7 ft. high, 

 which Joule, when working with the pump, clamped to the edge of 

 a table. 



The method of working the pump is first to raise the globe, d, 

 to the upper position, d l9 whereupon the mercury runs down, fills the 

 vessel, b, lifts the bulb at c, and fills the upper part of the thistle- 

 glass, driving the air out by the exit tube e ; cZ is then lowered to the 

 position d 2 , whereupon the mercury runs out of b back into d, and b 

 again fills with air from the entrance tube ; this operation is repeated 

 until the pressure in the manometer reaches the lowest limit obtain- 

 able with the pump. The funnel, k, is used to introduce sulphuric 

 acid for removing aqueous vapour. 



Joule gives the following list of lowest pressures obtainable with 

 sulphuric acid of various strengths : — 



