THERMAL CONDUCTIVITY OF METALS. 97 



as for example at A and B (fig. 8), it is clear such fissures will open outwardly ; 

 but in the interval of two such fissures, as at C, another must be found opening, 

 on the contrary, downivards, that is, towards the confined and heated lava, 

 which in consequence must intumesce and fill the space afforded to it, and 

 perhaps force its way through some minor cleft upon the external surface of 

 the elevated rocks." 



Plato we know to be a large cavity in an elevated region, between the 

 Mare Imbrium and the Mare Frigoris, connected with the mountain-studded 

 region of the Alps on the west, and descending with a precipitous slope 

 towards the east. The whole of the surface around Plato is exceedingly 

 rugged, containing at least the remains of three craters of more ancient date. 

 It is the floor of Plato only that presents any appearance of a recent character ; 

 and even this when Aaewed by very obhque light is far from being level. 

 The sketch (fig. 8) to which reference has already been made is intended to con- 

 vey some idea of the successive steps by which it is probable that Plato has 

 arrived at its present form. It is roughly drawn to scale, which is somewhat 

 too small, and, consequently, the height of the rim rather exaggerated ; the 

 extent being 316,800 English feet, the height, under 4000 feet (i. e. of the 

 rim exclusive of the four pinnacles), will be nearly -j^jth part. The letters 

 A and B are placed over the supposed foci of expansion, the arrows indi- 

 cating the direction of the elevating movements, the dotted line showing the 

 extreme height to which the surface could be raised without fracture. Over 

 A and B, and above C, are placed the three main fissures resulting from the in- 

 creased tension and the general breaking up of the elevated mass, and which 

 might have been accompanied with an almost immediate subsidence, as sug- 

 gested by Hopkins, Eeport Brit. Assoc. 1847, p. 64, in the following passage : — 

 " If the intumescence of the subjacent fluid, and consequently its supporting 

 power, were immediately afterwards diminished by the escape of elastic 

 vapours, there would be an immediate subsidence." Such a subsidence, or 

 rather a succession of subsidences, would fuUy account for the formation of 

 the floors of most craters ; and the upwelling of lava from numerous small 

 orifices would tend to produce such a floor as we observe on Plato. The 

 section presents all the characteristics of the walled plain under considera- 

 tion, the dip towards the border being strongly indicative of the main line of 

 fissiire opening outwardly at the foot of the rampart. It may be well to 

 mention that no new principle is introduced in this explanation, which is 

 based upon the views of two leading geologists, after comparing them with 

 phenomena that have been assiduously and repeatedly observed. 



Second Provisional Report on the Thermal Conductivity of Metals. 



By Prof. Tait. 



Since the date of the former Report the Committee have obtained a splendid 

 set of Kew standard thermometers. "With these, complete sets of observa- 

 tions, at very diiferent temperatures, have been made on iron, two specimens 

 of copper, lead, german silver, and gas-coke. As great difficulty was found 

 in keeping the source of heat at a constant high temperature in the statical 

 experiments, they were repeated from day to day tiU satisfactory results 

 were obtained. But a simple and ingenious device of Dr. Crum Brown (con- 

 sisting in making the descending counterpoise of a small gas-holder nip an 

 india-rubber tube) supplied so very great an improvement in steadiness of 

 temperature that it was considered advisable to repeat all the statical expe- 

 1871. H 



