BODIES IMMERSED IN LIQUIDS. 187 



)mewhat greater height than that in accordance with the law 

 previously enunciated. 



For the experiments with two liquids water is used, with some 

 liquid which does not mix with it, as mercury, ether (the so-called 

 sulphuric ether), olive-oil, or paraffin-oil which is better than any of 

 the others. Mercury (sp. gr. 13'6) and ether (sp. gr. O74) differ 

 greatly in their specific gravity from that of water, and do not soil 

 the vessels ; but both are rather expensive, and the mercury is not 

 easily poured into the somewhat inconvenient communicating 

 vessels without some of it being lost, while the ether is as colourless 

 as water and not easily distinguished from it at a distance. Olive- 

 oil soils the vessels very much, and as its sp. gr. is 0'9 and more, its 

 height in the vessel is very little greater than that of the water. 

 Paraffin-oil is lighter and leaves the vessels clean after evaporation, 

 especially the lighter kinds sometimes named petroleum- ether, 

 petroleum-naphtha, or what is sold under the name of benzine for 

 removing spots from articles of dress. The specific gravity of these 

 kinds of petroleum varies from 0'8 to 0'7, and even somewhat less. 



The heavier liquid is always poured in first. The separation of 

 both liquids after the experiment involves almost always some loss. 

 If water and paraffin- oil have been used, it is best to pour them 

 together into a larger vessel and decant the oil carefully into the 

 bottle, in which it is to be kept, through a funnel. Some of it is, 

 however, always lost. 



* A body wholly immersed, as in fig. 135, is pressed by 

 the liquid on all sides. It will easily be perceived 

 that the pressures upon the right and 

 left side of the body represented are 

 in equilibrium; this is also the case 

 with the pressures upon the two other 

 vertical sides of the body, viz. that in 

 front and that behind. On the other 

 hand, the pressure upon the lower 

 surface a b is equal to the weight of 

 a column of water represented by 

 n b ef, while the pressure upon the upper surface c d 

 i.s equal to the weight of the smaller column of liquid 

 cdef. The pressure upon the upper surface is there- 



