8 REPORT 1876. 



on until the sunk portions of crust build up from the bottom a sufficiently close- 

 ribbed skeleton or frame to allow fresh incrustations to remain, bridging across 

 the now small areas of lava pools or lakes. 



" In the honeycombed solid and liquid mass thus formed there must be a con- 

 tinual tendency for the liquid, in consequence of its less specitic gravity, to work 

 its way up ; whether by masses of solid falling from the roofs of vesicles or tunnels 

 and causing earthquake-shocks, or by the roof breaking quite through when very 

 thin, so as to cause two such hollows to unite or the liquid of any of them to flow 

 out freely over the outer surface of the earth, or by gradual subsidence of the solid 

 owing to the thermodynamic melting which portions of it under intense stress 

 must experience, according to my brother's theory. The results which must follow 

 from this tendency seem sufficiently great and various to account for all that we 

 learn from geological evidence of earthquakes, of upheavals and subsidences of 

 solid, and of eruptions of melted rock."* 



Leaving altogether now the hypothesis of a hollow shell filled with liquid, we 

 must still face the question, how much does the earth, solid throughout, except 

 small cavities or vesicles filled with liquid, yield to the deforming (or tide-gene- 

 rating) influences of sun and moon ? This question can only be answered by obser- 

 vation. A single infinitely accurate spirit-level or plummet far enough away from 

 the sea to be not sensibly affected by the attraction of the rising and falling water 

 would enable us to find the answer. Observe by level or plummet the changes of 

 direction of apparent gravity relatively to an object rigidly connected with the 

 earth, and compare these changes with what they would be were the earth perfectly 

 rigid, according to the kno'WTi masses and distances of sun and moon. The dis- 

 crepance, if any is found, wo idd show distortion of the earth, and would afford data 

 for determining the dimensions of the elliptic spheroid into which a non-rotating 

 globular mass of the same dimensions and elasticity as the earth woidd be distorted 

 by centrifugal force if set in rotation, or by tide- generating influences of sun or 

 moon. The effect on the plumb-line of the lunar tide-generating influence is to 

 deflect it towards or from the point of the horizon nearest to the moon, according 

 as the moon is above or below the horizon. The effect is zero when the moon is 

 on the horizon or overhead, and is greatest in either direction when the moon is 

 46° above or below the horizon. "VVheu this greatest value is reached, the plummet 

 is drawn from its mean position through a space equal to ., f.(,n nnn ^^ ^^® length of 

 the thread. No ordinary plummet or spirit-level could give any perceptible indication 

 whatever of this effect ; and to measm-e its amount it would be necessary to be able 



to observe angles as small as ivp qqo ooo °^ ^^^ radian, or about -g^". Siemens's 

 beautiful hydrostatical multiplying level may probably supply the means for doing- 

 this. Otherwise at present no apparatus exists within small compass by which it 

 could be done. A submerged water-pipe of considerable length, say 12 kilometres, 

 with its two ends turned up and open, might answer. Suppose, for example, the 

 tube to lie north and south, and its two ends to open into two small cisterns, one 

 of them, the southern for example, of half a decimetre diameter (to escape disturb- 

 ance from capillary attraction), and the other of two or three decimetres diameter 

 (so as to throw nearly the whole rise and fall into the smaller cistern). For sim- 

 plicity, suppose the time of observation to be when the moon's declination is zero. 

 The water in the smaller or southern cistern will rise from its lowest position to 

 its highest position while the moon is rising to maximum altitude, and fall again 

 after the moon crosses the meridian till she sets ; and it will rise and fall again 

 through the same range from moouset to moonrise. If the earth were perfectly 

 rigid, and if the locality is in latitude 45°, the rise and fall would be half a milli- 

 metre on each side of the mean level, or a little short of half a millimetre if the 

 place is within 10° north or south of latitude 45°. If the air were so absolutely 

 qiuescent during the observations as to give no varying differential pressure on 

 the two water-surfaces to the amount of -j^g millimetre of water or ^ f\g of mer- 

 cury, the observation would be satisfactorily practicable, as it wotild not be difficult 



* " Secular Cooling of the Earth, " Transactions of the Eoyal Society of Edinburgh, 

 1862 (W.Thomson), and Thomson and Tait's ' Natural Philosophy,' §§ (ec), (ff). 



