Improvement of the Barometer. 101 



the cistern,) the air strikes the end of the naercurial cokimn, and 

 must rise in the tube, because it is lighter. Secondly, by a con- 

 cussion which it receives, every few minutes while in use. eiiher 

 from the motion of the ship, the carriage, the shrubbery on a moun- 

 tain, or the unavoidable contact with the car and cords in a balloon ; for 

 by observing the mercury in a glass cistern, you will perceive that a 

 concussion causes a motion like the sea waves, which mounting on 

 one side, frequently leaves the end of the tube exposed to the atmos- 

 phere, which here strikes the base of the column and rises in the 

 tube by its comparative weight. Thirdly, it is asserted that the ba- 

 rometer, in a course of years, will have accumulated air above the 

 column, even if during all that time, it should iiave been suspended 

 in a room, without any jar or concussion to communicate the least 

 motion ; and the two most probable causes assigned are, first, that the 

 air enters through the pores of the tube, and secondly, that mercury 

 never comes into perfect contact with glass ; the latter is the most 

 probable cause, from which it is inferred, that the air in the cistern, 

 is by the atmospheric pressure forced down, in extremely minute par- 

 ticles between the mercury and tube, where it acquires the addition- 

 al impetus of its own comparative specific gravity, and rises between 

 the mercury and internal surface of the tube to the top of the col- 

 umn. As a preventive to the latter derangement, it has been sug- 

 gested, and I believe practiced by some, to fit closely on the bottom 

 of the tube, a ring of platina or any other substance with which mer- 

 cury comes in perfect contact, although without sufficient action to 

 cause, for years, any perceptible diminution. 



From the important purposes to which the barometer is adapted, 

 it may well be supposed to have enlisted the attention of the most 

 scientific men in all countries, and indeed for some of its uses it is 

 invaluable, and probably no instrument will ever be invented, with 

 any proportion of its combined properties. For although by a 

 number of instruments, we can weigh the atmospheric pressure, yet 

 even if the instruments would give the precise weight, the time oc- 

 cupied to obtain the result, would render useless the object for which 

 the trial was made, as the wind or calm would have actually arrived 

 which was predicted by the state of the atmosphere when the baro- 

 metrical observation was made. 



A gentleman commanding one of the New York and Havre Pack- 

 ets, for whose scientific knowledge I entertain a high regard, told 

 roe, " that when the ship was moving with much velocity, even the 



