PNEUMATICS. 



above the mercury should be a perfect 

 vacuum ; for if it be occupied by any 

 fluid, this will press on the surface of the 

 mercury in the tube, and the real weight 

 of the atmosphere will be equal to that 

 of the mercury suspended in the tube, 

 together with the pressure of the fluid 

 above it. 



To prevent the existence of any air 

 or other fluid in this space, two precau- 

 tions are necessary : first, the mercury 

 must be well purified before it is intro- 

 duced into the tube ; and, secondly, the 

 interior of the tube itself must be ren- 

 dered perfectly clean. 



Mercury, in its ordinary state, gene- 

 rally contains a quantity of air incorpo- 

 rated with it, or fixed in it : by boiling 

 it, this air is rendered highly elastic, and 

 it accordingly disengages itself, and 

 escapes at the surface. If this air were 

 permitted to remain in the mercury 

 when introduced into the tube, it would 

 escape from it when the mercury would 

 be relieved from the atmospheric pres- 

 sure by inverting the tube in the cistern, 

 and would rise in bubbles at the top, and 

 thus would occupy the part of the tube 

 above the mercury, and exert a pressure 

 on its surface an effect which is parti- 

 cularly to be avoided. 



Particles of moisture and air are apt 

 to adhere to the interior surface of the 

 tube : these are also removed by heat. 

 On introducing the cleansed mercury 

 into the tube, it will even contribute to 

 the perfection of the instrument to boil 

 it in the tube : this, which can be done 

 without much difficulty, will at once dis- 

 engage from it the particles of air and 

 moisture which may remain either in 

 the mercury itself or on the tube. 



(17.) In estimating the weight or 

 pressure of the atmosphere by a baro- 

 metric tube thus prepared, it is neces- 

 sary to measure the height E of the 

 mercury in the tube, above the surface 

 F of the mercury in the cistern. There- 

 fore, the graduation of the scale by which 

 this height is measured, should be taken 

 from the surface of the mercury in the 

 cistern. It is found that the weight of 

 the atmosphere is subject to very irre- 

 gular changes, being at one time capable 

 of sustaining a greater column of mer- 

 cury than at another ; and one of the 

 most common and interesting uses of 

 the barometer is to mark these changes. 

 The weight of the atmosphere is never 

 less than what sustains a column of 

 mercury of twenty-eight inches, nor 

 greater than what supports one of thirty- 



one inches. Thus, supposing the baro- 

 metric tube to be perpendicular, the sur- 

 face of the mercury in it has a range of 

 three inches. 



- This variation cf the height of the 

 mercury in the tube, united with the 

 necessity of measuring the column from 

 the surface of the mercury in the cistern, 

 suggests a circumstance which should 

 be attended to in the construction of a 

 barometer. 



As the surface E of the mercury falls 

 in the tube, the surface C F of the mer- 

 cury in the cistern must necessarily rise ; 

 since the mercury, which is discharged 

 from the tube, is thrown into the cistern : 

 and, on the other hand, when the surface 

 E of the mercury in the tube rises, that 

 C F of the mercury in the cistern must 

 fall. Thus the two surfaces move always 

 together, and in opposite directions. 

 It must, therefore, be evident, that if 

 the scale, by which the distance between 

 these surfaces is measured, be fixed, 

 two observations would be necessaiy to 

 determine the height of the column. To 

 avoid this, the cistern is usually con- 

 structed of very considerable dimensions, 

 in comparison with those of the tube ; 

 so that the cubic quantity of mercury 

 contained in three inches of the tube, 

 can produce a very inconsiderable 

 change in the level of the mercury in 

 the cistern. 



(18.) Such a barometer, although suf- 

 ficient for the more popular purposes, 

 does not give all the accuracy requisite 

 for some more scientific investigations. 

 Accordingly, contrivances have been 

 adopted for regulating the level of the 

 mercury in the cistern, among which the 

 following is worthy of notice : 



The glass tube is enclosed in another 

 of brass, in which, however, a longitu- 

 dinal aperture DE (Jig. 6.) is cut, ex- 

 tending from the lowest to beyond the 

 highest altitude of the mercury ; so that 

 the whole play of the barometer is in- 

 cluded between D and E. The cylindri- 

 cal cistern A B, in which the tube is 

 plunged, has a bottom B moveable by 

 a screw V ; so that it may be raiseci 

 and lowered at pleasure, and a corre- 

 sponding motion given to the level of the 

 mercury in the cistern. An ivory index 

 is attached to the top of the cistern, 

 with a fine point P, which marks the 

 level from which the divisions of the 

 scale CF are measured. When the height 

 of the barometric column is to be ob- 

 served, the screw V is to be turned, un- 

 til the point P meets the surface of the 



