The Variation in Attraction Due to the Attracting Bodies. 249 



The component of the moon's attraction that produces the ocean ti-les, 

 is only a small fraction of the whole, and this component decreases to 

 nothing at the center of the ear h. The slowness of the action of this 

 component force and the cohesion of tlie p irticles of water are such that 

 the tiiie at any meridan does not appear till two hours after the moon has 

 passed that meridan. If the tidal work was done instantaneously the 

 tide would appear directly under the moon. A less component of attrac- 

 tion and a greater c hesion of particles results in slower tidal work. Such 

 a decrease in the component attractions and such an increase in the cohe- 

 sion of the particles from the surface of the earth to its center can be con- 

 ceived from which the tidal tffects would be wholly neutralized. It is at 

 lea^t evident in case of days of one hour instead of twenty-four, suppos- 

 ing no change of figure from change of rotation, that the ocean tides 

 would gl'de to the condition of no tides as effectually as a boy slides over 

 a thin scale of ice. The astronomer seems to have forgotten that it takes 

 time to break ice. What is true of a water ocean and dajs of one hour 

 is true of a plastic ocean and days of twenty-four hours. A crust that 

 would be perceptibly flexible if the day were a month, might be imper- 

 ceptibly so for a day of twenty-four hours. In case then, of the one 

 hundred and twenty seven miles of solid rock crust and the plastic moul- 

 ten interior what more could be reasonably expected for tides on the 

 continents than infinitesimal vibrations. 



31. The density or sp cific gravity of tcie original crust of the earth at 

 the temperature of Centigrade. 



Tue order of bvolution is from a condition of homogeneity to that of 

 heterogeneity. To meet the results of the requisite test observations and 

 experiments, the computations of the last article require the plastic inte- 

 rior of the earih to be homogeneous excepting the effect from the varia- 

 tion ©f pressure. That portion of the primitive crust no; yet disintegrated 

 by the action of water or otherwise is likewise homogeneous, and the re- 

 sults of ihe best investigations point it out to be a neutral rock, largely 

 basic, sucli a rock as would be formed from the disintegrated material, not 

 includii g tlie elements gathei'ed in from the pristine heterogeneous atmos- 

 phere. Lyell, in speaking of trappean or volcanic rocks, says: "Abich has, 

 therefore, proposed that we weigh these rocks in order to appreciate their 

 composition in cases where it is impossible to separate their component 

 minerals. Thus basalt fiom Staffa, containing 47.80 per cent, of silica, has 

 a specific gravity of 2.9"); whereas trachyte, which has 66 per cent, of silica, 

 has a sp. gr. of only 2.CS; trachytic porphyry, containing 69 per cent, 

 of silica, a sp. gr of only 2.58, If we take a rock of intermediate com- 

 position, such as that prevailing at the Peak of Teneriffe, which Abich 

 calls Trachyte-dolerite, its proportion of silica being intermediate, or 58 per 

 cent, it weighs 2.78."' On this principal a trappean rock thrown up from a 

 depth of 38 per cent, of silica would have a specific gravity of 3.12. 



For each degi-ee Centigrade granite expands in volume .000026, marble 



