THE SYMPIESOMETEU. 



48& 



consists in employing an elastic fluid or gas, diffe- 

 rent from air, and any liquid, excepting quicksilver, 

 which neither acts upon the gas which it confines, 

 nor is perceptibly acted upon by the air, to the con- 

 tact of which it is in some measure exposed. Hy- 

 drogen gas, azotic gas, or any of the gases not lia- 

 ble to be absorbed by the inclosing fluid, may be 

 used ; but I prefer hydrogen gas as superior to any 

 other that I have tried. The liquid which answers 

 best is an unctuous oil, or a mixture of unctuous 

 and volatile oils. I consider almond oil, coloured 

 with anchusa root, as the most eligible. 



The Sympiesometer consists of a tube of glass A 

 B C, of about 18 inches long, and 0.7 of an inch 

 diameter inside, terminated above by a bulb A, 

 about two inches long inside, and half an inch dia- 

 meter, (but this will vary, as the instrument is re- 

 quired to have a greater or lesser range) ; and ha- 

 ving the lower extremity B bent upward, and ex- 

 panding into an oval cistern C„ open at top. 



The bulb A at the upper end of the tube is 

 drawn to a slender thread, and is at first left open. 

 In order to introduce the gas and oil, I fill the bulb 

 and tube with quicksilver : Then holding the tube 

 horizontal, a communication is formed between a 

 gasometer, containing the gas to be used, and the 

 slender pipe at the end of the bulb A, by means of 

 a flexible tube. As the tube is brought to a verti 

 cal position, the quicksilver flows out till it descends 

 in the tube to the level of the top of the cistern, 



