135 



The author then enters fully into the details of the methods he em- 

 ployed for constructing the whole of the apparatus, and for placing 

 it in its present situation in the centre of the winding staircase con- 

 ducting to the apartments of the Royal Society. The tube was very 

 skilfully made by Messrs. Pellattand Co. at the Falcon Glass-house. 

 It was 40 feet long, and one inch in diameter at its lower end; and 

 so nearly cylindrical, throughout its whole extent, as to diminish only 

 by two 'tenths of an inch at its upper end. A second tube of the 

 same dimensions was also made as a provision in reserve against any 

 accident happening to the first. These tubes were both securely 

 lodged in a square case by means of proper supports. A small ther- 

 mometer with a platina scale, was introduced into the upper end of 

 the tube. An external collar of glass was united to that end byheat- 

 ing it. This was done with a view of giving it additional support, 

 and of preventing it from slipping. This end of the tube was then 

 drawn out into a fine tube ready for sealing with the blowpipe; and 

 a small stopcock was fitted on to it. The cistern of the barometer 

 was formed by a small copper steam boiler, lcS inches long, 11 wide, 

 and 10 deep, capable of being closed by a cock, and having at the 

 bottom a small receptacle for holding the lower end of the tube, so 

 as to allow of the water in the cistern being withdrawn, without dis- 

 turbing that contained in the tube. 



The boiler was set with brickwork, in a proper position, over a 

 small fire-place. It was nearly filled with distilled water, which was 

 made to boil thoroughly so as to free it from air; and the cock being 

 then closed, the water was raised in the tube by the pressure of the 

 steam collected in the upper part of the cistern. The tube, when 

 filled, was hermetically closed at the top : a proper scale, constructed 

 by Newman, was applied to it, great care being taken to determine 

 its height and to ensure the accuracy of its adjustments, and the 

 precision of its measurements, by an exact mode of reading; and 

 also to provide proper corrections for temperature. The water in 

 the cistern was protected from contact with the air by being covered 

 with pure castor oil to the depth of half an inch. The mercurial ba- 

 rometer employed as a standard of comparison, was of a portable 

 construction, and was provided with a platina guard. 



An account is then given of some of the results of the observations 

 made with this water barometer, arranged in several sets of tables. 

 Tne great object was to obtain good and uninterrupted series of ob- 

 servations, taken, at least once a day, at a fixed hour. The regis- 

 ters given by the author, contain such observations, continued for 

 nearly a year and a half, namely, from October 1830 to March 

 1832. Some curious results are afforded by these observations. In 

 windy weather the column of water is found to be in perpetual mo- 

 tion, not unlike that from the breathing of an animal. Many consi- 

 derable fluctuations in the pressure of the atmosphere are rendered 

 sensible by the motions of an aqueous column, which would totally 

 escape detection by the ordinary mercurial barometer. Mr. Hudson 

 remarked in the course of his observations, that the rise and fall of 

 the water barometer precedes by one hour the similar motions of the 



