HYDROSTATICS. 123 



nent parts, is more readily decomposed by the action of moisture and frost 

 than the quartz, which is another ingredient ; and therefore it is very unsuit- 

 able for building purposes. Moreover, if it possess an iron-brown or rusty 

 appearance, it may be set down as highly perishable, owing to the attraction 

 •which tliis iron has for oxygen, causing the rock to increase in bulk, and so 

 disintegrate. 



Sai^dstones, termed freestones, are ill adapted for the external portions of 

 exposed buildings, because they readily absorb moisture ; and in countries 

 ■where frosts occur, the freezing of the water on the wet surface continually 

 peels off the external portions, and thus, in time, all ornamental work upon 

 tne stone will be defaced or destroyed. 



CHAPTER VIII. 



HYDROSTATICS. 



What is the 285. Hydrostatics is that department of 



science of Hy- ^ ^ ^ 



drostatics? Physical Science which treats of the weight, 

 pressure, and equilibrium of water,* and other liquids at 

 rest. 



• Water is a fluid composed of oxygen and hydrogen, in the proportion of 8 parts of 

 oxygen to 1 of hydrogen. It is one of the most abundant of all substances, constituting 

 three fourths of the weight of living animals and plants, and covering about three-fifth* 

 of the earth's surface, in the form of oceans, seas, lakes, and rivers. 



In the northern hemisphere the proportion of land to water is as 419 to 1000; while iq 

 the southern hemisphere it is as 129 to 1000. The maximum depth of the ocean has never 

 been ascertained. Soundings were obtained in the South Atlantic iu 1S53, between Rio 

 Janeiro and the Cape of Good Hope, to the depth of 48,000 feet, or about 9 miles. Other 

 soundings, made during the recent U. S. survey of the Gulf Stream, extended to the 

 depth of 34,200 feet without finding bottom. The average depth of the ocean has been 

 estimated at about 2000 fathoms. 



Notwithstanding this apparent immensity of the ocean, yet, compared with the whole 

 bulk of the earth, it is a mere film upon its surface ; and if its depth were represented on 

 an ordinary globe, it would hardly exceed the coating of varnish placed there by the 

 manufacturer. 



The source of all our terrestrial waters is the ocean. By the action of evaporation upon 

 its surface, a portion of its water is constantly rising into the atmosphere in the form of 

 vapor, which again descends in the form of rain, dew, fog, etc. These waters combine to 

 form springs and rivers, which all at last discharge into the ocean, the point from which 

 tUoy originally came, thus forming a constant round and circulation. " All the rivers 

 run into the sea, yet the sea is not full," because the quantity of water evaporated from 

 the sea exactly equals the qu!\ntity poured into it by the rivers. In nature, water is 

 never found perfectly pure; that which descends as rain is contaminated by the impuri- 

 ties it washes out of the air ; that which rises in springs by the substances it meets with 

 in the earth. Any water which contains less than fifteen grains of solid mineral matter in 

 a gallon, is considered as comparatively pure. Some natural waters are known so pure 

 that they contain only l-20th of a grain of mineral matter to the gallon, but such instancoa 

 are very rare. Water obtained from different soarces may be classed, as rpga^-ds coc^- 



