56 KEPORT — 1861. 



as to whetlier this increase contiuues to mucli greater depths than we have yet 



Eeneti'ated, as observations upon this point are still imperfect. But, assuming as an 

 ypothesk that the law which prevails to a depth of 700 yards continues to operate 

 at still gi-eater depths, we arrive at the conclusion that at a depth of less than two 

 and a lialf miles the temperature of boiling water would be reached, and at a depth 

 of 40 miles a temperature of 3000° Fahrenheit, which we may assume to be sufficient 

 to m'felt the most refractory rocks of which the earth's crust is composed. If, 

 therefore, no other circumstance modified the conditions of liquefaction, all within 

 a thin crust of this thickness would be in a fiuid state. This, however, is not the 

 case. At these depths the fusing-point is modified by the pressure and conductivity 

 of the rocks. 



We know that in volcanic districts, where the great subterranean laboratory of 

 natm-e is partially opened for our inspection, the molten mass, relieved fi-om pres- 

 siu-e, pours forth from volcanic craters ciuTents of lava which form a peculiar class 

 of rocks. 



Besides this, it has been ascertained by experiment on soft substances, such as 

 spermaceti, wax, and sulphur, that the temperatiu'e of fusion increases about l°-3 

 Pahi'enheit for every 500 lbs. pressure per square inch, — that is, in other words, that 

 the temperature of fusion under pressure is increased in that ratio. If we assume 

 this to be the law for the materials of the earth's crust, and correct our previous 

 calculations in accordance with it, we find that we shall have to go to a depth of 

 65 miles, instead of merely 40 miles, before the poiat of fusion of the rocks is 

 reached. 



It must, however, be observed that Mr. Hopkins's later experiments with tin and 

 barytes do not show such an increase of the point of fusion m consequence of pres- 

 sure, and he is led to the belief that it is only in the more compressible substances 

 that the law holds true. 



Independently of this, however, I\Ir. Hopkins points out to me that in the above 

 calculation it is assumed that the conductivity of the rocks is the same at great 

 depths as at the surface. In opposition to this he has shown experimentally that 

 the conducting power for heat is at least twice as great for the dense igneous 

 rocks as for the more superfcial sedimentary formations of clay, sand, chalk, &c. 

 And these close-grained igneous rocks are those which we believe must most 

 resemble the rocks at great depths below the sm-face. Now Mr. Hopkins shows 

 that if the conductive power were doubled, the increase of depth, corresponding to 

 a given increase of temperatm-e, would be doubled, and we should probably have to 

 descend 80 or 100 miles to reach a temperature of 3000^, besides the further increase 

 which investigation may show to be due to the influence of pressure on the tempe- 

 rature of fusion. 



Mr. Hopkins therefore concludes that the exti-eme thinness of the criist assumed 

 by some geologists to account for volcanic phenomena is imtenable. Calculations 

 on entirely independent data led him to conclude that the thickness did not fall 

 short of 800, instead of 30 or 40 miles. If it be so much, he is fiuther led to be- 

 lieve that the superficial temperature of the cnist is due to some other cause than 

 an internal fluid nucleus. It remains a problem, therefore, which my friend Mr. 

 Hopkins is endeavouring to solve, as to what is the actual condition of the earth 

 at great depths, and the relation of terrestrial heat to volcanic phenomena. 



Tidal Observations. By Rear- Admiral FitzEot, F.R.S. 



Since the publication of Dr. WheweU's invaluable essays on Tides, much addi- 

 tional information has been collected by the Admiralty, tMough various siu-veying 

 expeditions in many parts of the world, respecting tides. 



The accompanying volume of tide-tables shows to what extent our acquaintance 

 with the facts of the subject goes at present. 



However extended a Imowledge of tidal facts may be wow, compared with that of 

 the earlier of those past years (some thu-ty), in which all maritime nations have 

 benefited from light thrown on the subject by that "Essay towards an approxima- 

 tion," which enabled seamen to discriminate between features until then viewed 

 in only a confused manner, and taught them clearly how and what to obsei-ve, 

 there is still very much to be learned. 



The useful, indeed now indispensable, yearly volume published by the Hydro- 



