84 Aneroid Barometer. 



from the account of the iustrnment given in the transactions 

 of the British Association at its first introduction. 



The mechanical arrangement consists of a cylindrical 

 chamher of copper about 2f inches in diameter and -^^ of an 

 inch deep, partially exhausted of air, and hermetically 

 sealed : the surface of this chamher is of very thin metal, 

 co-rrugated or in folds ; and -with every variation of the 

 atmosphere this surface rises or falls. The movement of the 

 surface is communicated by a strong stud fixed in its centre, 

 which acts upon a main lever attached to it, and the move- 

 ment is continued by a system of levers, from which a chain 

 connected with a spiral spring changes tlie motion from 

 vertical to horizontal, the spring being fixed to an axis, on 

 which is placed the steel hand or index indicating the 

 changes on the dial plate : thus, the chamber is a substitute 

 for the Torricellian tube, and its surface for that of the column 

 of mercury. It is therefore obvious that the difficulty in 

 first adjusting this instrument consists in the delicate 

 manipulation necessary to arrange the movement of the 

 levers and surface of the cylinder in connection with the 

 quantity of air to be withdrawn from the chamber, and can 

 only be accurately done in conjunction with some authentic 

 mercurial barometer, which alone furnishes the true weight 

 of a column of the atmosphere at any moment. Here it is 

 that I conceive the principle of the instrument to be 

 objectionable, and particularly so when employed in the 

 mensuration of heights, on account of the variation which 

 different altitudes on the earth's surface would cause in the 

 density of the air contained in the chamber ; because the 

 density of air is affected not only by changes of temperature, 

 but by changes in the weight of the incumbent atmosphere. 

 The comparisons which have been made with the mercurial 

 standard barometer of the Royal Observatory are annexed 

 in the form of a diagram, as more readily conveying the 



