Mr. Nixon's TJieory of the Spirit-Level. 355 



contained liquid, the following experiments were made with 

 a straight glass tube having a bore of 0*5 inch. Both ends 

 being stoppered, an open- 

 ing a, of an irregular fi- 

 gure, about 0*2 in. across 

 and 0*3 in. deep, was made 

 in a part of the tube equi- 

 distant from the ends. The 

 tube being placed horizon- 

 tally with the orifice up- 

 wards, was filled with wa- 

 ter, which stood within it 

 apparently the same in vo- 

 lume and figure as though the tube had been entire and im- 

 pervious to the air. Drawing out the stoppers, so as gradu- 

 ally to augment the space between them, the atmospheric air 

 soon made its ingress, causing that portion of the water im- 

 mediately under the orifice on which it -was, incumbent to as- 

 sume the concave surface, of which a section is given in the 

 upper figure. But as the interior space continued to augment, 

 the bubble of air elongated towards the stoppers without ma- 

 terial addition to its depth ; its ends being equidistant from c, 

 and curved precisely as those of the bubble of a spirit-level. 

 On pushing the stoppers home, the bubble, repassing through 

 the same changes of figure, was finally expelled from the tube. 



The tube being well dried, the experiment was repeated 

 with mercury, which not only filled the tube, but stood at a 

 quite out of it. On increasing the interior space, the mutual 

 repulsion of glass and mercury gave room to the latter to 

 shrink in the first instance from the upper corners of the tube, 

 thus checking the admission of the air until a further addi- 

 tion of space within the tube caused the mercury to subside 

 throughout its length with a surface nearly horizontal, but 

 terminating at the stoppers in a convex curve as before*. 



In order to enable us to comprehend, in some degree, why 

 the water filling the horizontal tube, so soon as the containing 

 sfjace became enlarged, should not subside equally the whole 

 length of the tube, and come to rest with a horizontal surface 

 of which the bounding lines (a right-angled parallelogram) 

 would be in every point in contact with the sides of the tube 

 or ends of" the stoppers, it will be necessary to notice such of 

 the phainomena of capillary attraction as appear calculated to 

 illustrate the subject. 



(1). When a vertical cylinder or plate of glass is partly im- 



• Had the stoppers been of platinum, what would have been the figure? 



2 Z 2 mersed 



