Mechanical Science. 601 



rail resembles that which would be given by one without any irregu- 

 larities at all. The varieties of form and colour in the road and 

 banks are too great, and separated by too long intervals, to be effaced 

 by any possible speed, and therefore they continue to recede from the 

 carriage after the rail seems to have changed the direction of its 

 motion. A. A. 



6. A BAROMETER OF A NEW CONSTRUCTION. (Proposed by 

 M. Kupffer.} 



The great danger of particles of air entering into the vacuum of a 

 barometer while in use, and the difficulty of effectually expelling such 

 air by boiling the mercury, have induced M. Kupffer to propose a 

 construction of this instrument by which the error arising from this 

 source, instead of being removed, may be taken into account. The 

 cistern of the barometer is a hollow iron cylinder ; the bottom of 

 which can be raised and lowered at pleasure by means of a screw. 

 Besides the ordinary tube of the barometer, another smaller tube, 

 like the short arm of a syphon, proceeds from, and communicates 

 with the cistern, which is quite full of mercury. The height of the 

 mercury in the two tubes, the difference of which is the height of 

 the barometer, may be changed at pleasure by means of the above 

 screw ; and the difference of level is measured by a scale, one extre- 

 mity of which, a steel point, reaches down into the shorter tube, so 

 that it may be brought into contact with the surface of the mercury 

 in it. By the action of the same screw, the space above the mer- 

 cury in the long tube, which ought to be a vacuum, may conse- 

 quently be reduced at pleasure to any degree. Now, if we suppose 

 that this space contains some air, let the height of the barometer, 

 which was observed while the space was e, be = A, and let the pres- 

 sure of the air in the tube at the same moment be = p. After 

 reducing the space to the smaller quantity c', let the height of the 

 barometrical column be =r B, and the corresponding pressure of the 



o e 1 



air = p' ; then will be, by the law of Mariotte, - = and the 



p' e 



corrected height of the barometer = A + p z: B + p' ; whence 



A-B 

 we find the correction to be applied to A, viz. p e . Ifc=2e', 



~7~ 



or if the space at the second observation is exactly half of what it 

 was at the first observation, we shall have p = A B. Thus the 

 difference of the observed heights will, in that case, be exactly the 

 correction to be applied to the first height for the error arising from 

 the action of the air *. 



7. OCCULTATION. 



M. Tarkhanoff, the astronomer of an imperial Russian expedition for 

 * Petersburgh Transactions, 1830, 



