CHAMBERS'S INFORMATION FOR THE PEOPLE. 



following consideration : if we could conceive a 

 portion of the surface heaped up, as at ef\ besides 

 causing unequal pressure, and therefore disturb- 

 ance in the mass below, the particles on the 

 summit would have a tendency to flow down as 

 on an inclined plane. Particles of sand so heaped 

 up have this tendency ; and are only prevented 

 from spreading out into a perfectly level surface by 

 their imperfect mobility. 



It seems so natural that the surface of an 

 unbroken mass of liquid should be all of one 

 height, that we seldom think of it as requiring 

 explanation. But when the surface of a liquid is 

 not continuous, being contained in separate vessels, 

 with communication between them, it requires con- 

 sideration, and experience to convince us that the 

 several parts of the surface must in all cases stand 

 at the same level. A common tea-pot affords the 

 most familiar illustration of this truth. 



Fig. 14 represents six vessels communicating 

 with one another, or rather one vessel consisting 



Fig. 14. 



of six compartments of different shapes and sizes ; 

 and it can be shewn in the same way as with a 

 continuous mass of water, that the surface in all 

 the compartments must form part of the same 

 horizontal line AB. 



It is only when a sheet of water is stretched out 

 to an extent of several miles that the convexity 

 becomes conspicuous. It is very perceptible on 

 the ocean when a ship is approaching on the 

 horizon ; first, the masts and sails of the ship are 

 seen, and lastly, the hull. The convexity of the 

 land is not so conspicuous, in consequence of the 

 many risings and fallings in the surface. It is 

 only in extensive plains that the roundness can 

 be perceived in the same manner as at sea. 



The amount of the earth's convexity can be 

 calculated exactly. It is very nearly eight inches 

 in each mile ; that is, if PL (fig. 12) is a mile long, 

 the point L will be eight inches above the surface. 

 In constructing canals and railways, allowance 

 must be made for the convexity of the earth ; for 

 the channel of a canal, or a line of level railway, 

 does not form a straight line, but a curve ; it is not 

 a part of the tangent or PL, but of the curve PA. 



Levelling. The most common instrument for 

 determining levels is the spirit-level. It consists 

 of a glass cylinder, ac, slightly convex on the 

 upper side, and filled with 

 spirits, all except a small 

 space. It is placed for 

 protection in a wooden 

 Fig. 15. or brass case, the bottom 



of which is exactly parallel 



with the tube. If the instrument is laid on the 

 upper surface of a stone slab or a beam, and if 

 one end of the slab or beam is higher than the 

 other, the liquid in the tube will seek the lowest 

 position, and the air-bubble will move towards the 



230 



higher end ; when the bubble rests in the middle, 

 as at b, the surface is horizontal or level. To 

 ascertain levels at a distance, the spirit-level is 

 fixed below a small telescope, the tube of the 

 telescope and the glass tube being made exactly 

 parallel. The telescope being fixed on a stand, is 

 adjusted by screws until the air-bubble of the 

 spirit-level stands exactly in the middle, and is 

 thus made perfectly horizontal. A pole being 

 now set up at the spot where the level is to be 

 found, the telescope is directed to it, and the point 

 of the pole which appears in the centre of view 

 is in a horizontal line with the eye. If the distance 

 is short, this point may be considered as the level - T 

 but if the pole is as far as a mile off, the natural 

 level at which water would stand, if there were a 

 sheet of it all the way, will be eight inches below. 

 The difference between the apparent level and the 

 true level, as they are sometimes called, varies 

 for different distances, and may either be calculated 

 or taken from a table. 



BUOYANCY AND FLOTATION. 



When a solid is immersed in a liquid, it 

 displaces exactly its own bulk of the liquid. 

 This requires no demonstration ; as soon as 

 we conceive clearly what is meant, the truth 

 of the proposition is self-evident. Of course, 

 it is only of solids that are neither melted nor 

 penetrated by the liquid that the proposition 

 holds good. 



If a cubic inch of wood or of a metal is 

 wholly immersed in a vessel previously full of water, 

 the liquid that occupied the space where the solid 

 now is, must overflow ; or, if the vessel was not 

 full, the level of the liquid must rise. This affords 

 a ready means of measuring solids, even the most 

 irregular in shape. To ascertain what bulk of 

 solid metal there is in a gold chain : have a 

 cylindrical vessel with a base of known area 

 say a square inch filled to a certain height with 

 water ; drop in the chain, and note the rise of 

 the liquid. If it rise an inch, the chain contains 

 a solid inch of gold, and so for any fraction of an 

 inch. Archimedes is said to have been the first 

 to think of measuring solid bodies in this way, 

 and to have applied it in detecting the adulteration 

 of a gold crown. 



Every one must have experienced that a heavy 

 body, held in the hand, becomes lighter when it is 

 immersed in a liquid. We have now to consider 

 how this loss of weight arises, and what is its 

 amount. 



Conceive, first, that a body of water, AB, form- 

 ing part of the contents of a vessel, is separated 

 from the rest by an imaginary 

 film ; this water will clearly 

 remain suspended like any 

 other portion of the mass. 

 Now, as its weight is pressing 

 it downwards, there must be 

 an equal force acting upon it 

 upwards. That force arises 

 from the pressure of the sur- 



Fig. 1 6. 



rounding liquid, which is acting upon it in all 

 directions, upwards, downwards, and laterally. 

 The horizontal pressures have each an opposite 

 and equal resultant, and thus mutually destroy 

 one another ; but the under surface being at a 

 greater depth than the upper, there remains a 



