﻿Intelligence and Miscellaneous Articles. 403 



whence 



*<* --„„.»„ VqP +00 -PoO+ftQ 1 0-001293 

 dB Q h 1+at 760 



a 



gives us the measure of the delicacy of the instrument, which therefore 

 reaches the maximum about the horizontal position of the beam. 

 As, moreover, the greatest deflection only amounts to a few degrees, 

 the delicacy can be regarded as constant. On a certain assumption, 

 calculation gives for an increase in the pressure of 1 millim. a deflec- 

 tion of a few millimetres (4 to 5) of the scale. 



But the present apparatus not merely gives a determination of the 

 pressure of the atmosphere with at least as great a degree of accu- 

 racy as the mercury barometer, but appears also to have several 

 material advantages over this, as it is liable to far smaller errors. 



In every instrument which serves for the measurement of a vari- 

 able force it is essential that it require the movement of as small 

 masses as possible, and the more so the quicker the changes which 

 it is to indicate ; for under certain circumstances results greatly 

 varying from actual fact would be obtained. 



In the mercury barometer the movement of a column of liquid of 

 considerable weight is essential in indicating changes in the atmo- 

 spheric pressure ; this heavy column of mercury will only be set in 

 motion if the impulses are accumulated in such a manner that they 

 can overcome the resistance along the entire surface of the tube and 

 acquire a certain velocity. If the change in the pressure take place 

 with great velocity, the mercury barometer, in virtue of its inertia, 

 will never indicate the actual pressure, and never its highest or lowest 

 value. 



From its small size, as well as from the small resistance to its 

 motion, the above apparatus is far less exposed to this objection ; 

 and we shall probably be enabled by its means to follow such rapid 

 changes in the pressure of the atmosphere as in the ordinary baro- 

 meter can only be expressed by mean values. 



A further disadvantage of the ordinary barometer is that, as a 

 matter of fact, the Torricellian vacuum always contains mercury 

 vapour of small tension and also atmospheric air, and the more of 

 this the longer the instrument has been in use. And although a 

 correction may be introduced for the vapour of mercury, its action 

 upon the capillary depression, as well as the air in the Torricellian 

 vacuum, cannot be accurately allowed for. 



Finally, the use of the mercury barometer presupposes filling it 

 with mercury of a precisely definite specific gravity, which also pre- 

 sumes an impossible condition ; for chemically pure mercury cannot 

 be obtained without difficulty, and does not keep, but partially 

 oxidizes after a time. Now experiments have shown that the smallest 

 impurity in the barometric liquid has a material influence on the 

 capillary depression. 



Defects analogous to these the new instrument does not seem to 

 have, provided the beam be constructed with that care which is 

 usually bestowed upon delicate balances. 



