Mechanical Science. 371 



8. Variation of Thermometers. — MM. A. de la Rive and F. 

 Marcet, have also investigated the elevation of the mercury in 

 thermometers, which is due to the cause pointed out by Mr. 

 Flaug-ergues, (xiv. 441,) namely, the continued pressure of the 

 air on its external surface: and by opening the top of the thermo- 

 meter ; by submitting the instrument to condensed or rare at- 

 mospheres ; and by comparison with thermometers otherwise 

 constructed, have abundantly proved the effect due to this 

 power. These philosophers had occasion also to remark some 

 curious effects due to the absorotion and evolution of heat, by 

 the expansion and condensation of gases, which, however, we 

 cannot at this time further attend to, than by copying the 

 conclusions at the end of the memoir. 



1. That atmospheric pressure exerts an influence on the bulk 

 of thermometer bulbs. 2. That in experiments, where this effect 

 may influence the results, it is better to use thermometers open 

 at the top. 3. That certainly cold is produced in making a 

 vaccuum by the air pump, but in smaller quantity than was sup- 

 posed *. 4. That when gases enter an exhausted vessel, there 

 is at first a production of cold, and then of heat. 5. Ihat 

 various modifications may render the cold produced at the 

 moment of the entrance of air into a vaccuum, more intense. — 

 Bib. Univ. xxii. 265. 



9. On Variations of Barometers and Thermometers. — Sig. Bel- 

 lani has undertaken a series of experiments, to determine whether 

 the air or vapour, the last portions of which are found to remain 

 so obstinately in barometers and thermometers, is introduced 

 with the mercury, or is a portion of that which originally occu- 

 pied the tube before the introduction of the metal. The con- 

 clusion he comes to is, that it is always a portion of that which 

 previously adhered to the glass, and that mercury is utterly in- 

 capable of absorbing either air or moisture. The extraordinary 

 way in which air and water is held at it were in a him over 

 glass, is insisted upon, and reference made to many authors in 

 proof of it. The following, however, are more interesting, as 

 being some of the facts he advances to prove that the mercury 

 never contains either of these substances. Fill a barometer 

 tube and boil it very carefully ; then prepare a kind of funnel 

 made of a small capillary tube, which will reach through the 

 mercury in the barometer tube to the closed end, and is enlarged 

 at top; let it be recently made, so as to be dry, and intro- 

 duce it into the barometer tube; prepare some mercury by agi- 

 tating it in a bottle with water and air, then drying its surface 

 with bibulous paper, and afterwards passing it through paper 

 cones three or four times into dry vessels ; pour a little of this 



• It has been stated, tlnit when one of INI. r.reguet's nictallio thermome- 

 ters has been used, the diniiiiutiun of temperature has amounted to :)0°. — 

 Ei). 



