Mechanical Science. 379 



tained, by means of water, what space in the tube was occupied 

 by the contents of the measure I employed. The air, of course, ex- 

 panded as it rose th rough the mercury, and at the surface it occu- 

 pied about four and a half or five times the space it occupied under 

 the whole pressure of the atmosphere. It is to be recollected, it 

 was still exposed to some part of this pressure, the mercury having 

 descended several inches: one portion of air being introduced 

 after another, it necessarily occupied less and less space as the 

 mercury descended. When the mercury had descended to about 

 eighteen inches above the surface of that in the basin, the addition 

 which the measure of air made to the space between the mercury 

 and the upper end of the tube was only about equal to the bulk of 

 the air before its introduction. When the mercury had descended 

 to fifteen inches, this addition was one-fourth less than the bulk of 

 the air before its introduction ; and when the mercury had de- 

 scended to about five inches above that in the basin, the addition 

 was only about one half of this bulk. The cause of this apparent 

 shrinking of the air introduced, is evidently the increased con- 

 densation of the air previously in the tube, in consequence of the 

 descent of the mercury exposing it to a greater degree of atmo- 

 spherical pressure ; but I had expected to find that the result, on 

 the whole, would have been expansion, not contraction. 



I wished to ascertain what was the actual expansion of the air 

 introduced when the mercury stood at fifteen inches ; for this pur- 

 pose, I introduced the same measure of water; this I found en- 

 larged the space above the mercury by one-half of the bulk of the 

 water. Here the degree in which the air in the tube contracted, 

 was pretty nearly the same as when the air was introduced ; the 

 mercury standing only about a fifth or sixth of an inch higher 

 than in the latter case. If we overlook this slight cause of 

 greater compression, we shall have the following statement : — the 

 contraction of the air previously in the tube equal to half a mea- 

 sure; the additional space above the mercury, after the intro- 

 duction of the air, three-quarters of a measure. Thus, it would 

 appear, that when the air is relieved from about one-half of the 

 pressure of the atmosphere, it occupies only about one-fourth 

 more space than under the whole of its pressure. I am, Sir, 



Your obedient servant, A. B. 



We do not understand our Correspondent's conclusion. — Ed. 



7. On the Attraction manifested at sensible Distances by solid Swr- 

 faces moistened by, an d immersed in, a Liquid. By M. P. S. Girard.— 

 Experiment has long since proved that the surface of certain solid 

 bodies may be moistened by liquids which have not the property of 

 moistening the surfaces of other bodies. Thus, mercury, which has 

 the power of moistening many metals, and adhering to their sur- 

 face, has no such property with regard to glass, wood, and many 



