98 Proceedings of the British Association. 



under the exhausted receiver of the air-pump, and actually had the 

 barometer of the Royal Society so filled by Mr. Newman, under his 

 own superintendence ; but although an expert London working optician 

 might be found capable of executing successfully such a tube, yet few in 

 the country could hope for such an advantage ; and, in fact, although 

 he had attempted the process at Belfast, he had never succeeded. After 

 some consideration, the following simple mode of using the torrecellian 

 vacuum of the tube itself, instead of the air-pump, in filling it, occurred 

 to him. He heated the mercury as hot as it could be used, and fil- 

 led the tube, in the common way, to within half an inch of the 

 top ; then worked out, in the usual way, all air bubbles, as per- 

 fectly as possible ; filled up the tube to the top, and inverted it in a cup 

 of hot mercury, when it, of course, subsided, in the upper part of the tube 

 to the barometric height ; he then placed his finger on the mouth of the 

 tube, under the mercury in the cup, and lifted it out ; and, still holding 

 his finger tightly over the mouth of the tube, laid it flat on a table, 

 when the mercury in the tube soon lay at the under side of the tube, 

 leaving the upper part along the length of the tube void. Upon then 

 turning the tube slowly round, still keeping the finger on its mouth, 

 every spark of air was gathered up. He then placed the tube in an 

 upright position, with its mouth upwards, and, placing a funnel of clean 

 dry paper about the upper part, an assistant filled the funnel slowly, 

 with hot mercury, so as to cover the fingers. Upon slowly withdraw- 

 ing the finger, the mercury went gently in, and displaced almost per- 

 fectly the atmospheric air which had gathered into the void space. By 

 renewing the process which succeeded the previous washing of the air 

 out of the tube, once, or at most twice, a column of the most perfect 

 brilliancy was obtained. He had mentioned this simple method to Dr. 

 Robinson, of Armagh, who suggested that, to get rid of the damp and 

 greasiness of the finger, it would be better to cover the mouth during 

 the process with clean and dry caoutchouc ; and this was found a decided 

 advantage. The method of procuring an invariable surface in the cistern 

 was equally simple. From the imperfection of ids sight, it was an object 

 of much interest to him to have as few readings or adjustments depend- 

 big on sight as possible. He proposed, therefore, to divided the cistern 

 into two compartments, by a diaphragm of sheet iron or glass, brought 

 to a sharp edge at top. Into one of these compartments, the barometer 

 tube dips ; in the other is placed a plunger of glass or cast iron, which 

 can be raised or lowered by a slow screw movement. To prepare for an 

 observation, the plunger is first screwed down, by which it displaces 

 the mercury in one compartment, and raises its surface in the other 

 above the edge of the diaphragm; upon raising it slowly again, the 

 mercury drains off to the level of the edge of the diaphragm, thus, at 

 every observation, reducing the surface to a fixed level, 



