34 REPORT 1860. 



also greatly exceed that of previous experience ; still the insulating material em- 

 ployed remains the same, and is therefore liable to be affected by the same causes of 



failure. 



The frequent failure of gutta percha has given rise lately to several projects ot 

 substituting India-rubber and its compounds for the same, which, owing to the 

 higher insulating properties and lesser inductive capacity of India-rubber, and 

 above all, owing to its greater homogeneity and resisting power to effects of heat, 

 give promise of valuable results in making electric telegraphs less liable to failure. 

 The chief difficulty consisted hitherto in working India-rubber in such a way as to 

 obtain uniform and perfect coatings upon the conductor without injury to the con- 

 ductor itself. The authors have endeavoured to remove this difficulty in construct- 

 ing a covering machine, which they brought before Section G of the Association. 



They conclude, — *' We do not wish, however, to rest upon our individual efforts 

 for the further development of this important new branch of applied science. Our 

 object ill writing this communication has been to show that, although submarine 

 electric telegraphs have often failed, the experience gained has not been lost ; and 

 that in bringing the present stock of knowledge to bear upon the subject more com- 

 plete success may be ensured." 



Astronomy. 



On the Forms of certain Lunar Craters indicative of the Operation of a 

 peculiar degrading Force. By W. R. Birt, F.R.A.S. 



There are on the surface of our satellite three well-marked classes of lunar craters, 

 those that are more or less complete in the outlines of the mountainous rings by 

 which they are surrounded, having in many cases a somewhat deep interior, and 

 appearing as excavations on the surface of the moon. Cleomedes, Geminus, and 

 others in their neighbourhood are examples. We have also among the perfectly 

 surrounded craters those that have their rings somewhat considerably elevated above 

 the general level of the lunar surface. Tycho may be cited as the most perfect 

 instance of the raised craters. Both these kinds agree in a very important particu- 

 lar ; the surrounding ring (whatever may be the varying altitudes of different peaks, 

 or however certain portions may rise higher than ochers) is in this class complete ; 

 there is no evidence of the operation of the peculiar degrading force-, to which 1 shall 

 presently allude— certainly not to any very great extent— in breaking down any por- 

 tion of the surrounding annulus. 



A second class of lunar crater consists of those that, having the surrounding ring 

 complete, do not exhibit the depth of such craters above specified, or the gradual rising 

 from the general surface as seen so distinctly in Tycho ; they stand out as it were 

 above those portions of the surfaces of the moon where they occur— generally the 

 Maria— as if the smooth undulating plains had come quite up to the rings which rise 

 abruptly from them. Most of these craters have smooth level interiors ; and there 

 are instances of the first class situated in rugged mountainous districts possessing 

 also a smooth interior. Plato may be quoted as an example. Many instances of 

 this class occur in which the ring is but slightly raised above the interior and exte- 

 rior surfaces. 



The third class, to which I am particularly desirous of referring, consists of such 

 craters as having apparently at some previous period of their history possessed a 

 perfect ring ; a degrading force, not such as may have produced the terraces and 

 ravines which we notice in Copernicus, but something of a different character, has 

 invaded them from without, breaking down certain portions of the annulus, and 

 leaving only a portion of the walls standing : these craters mostly occur on the bor- 

 ders of the Maria ; and it is not a little significant that the broken portions are in- 

 variably, so far as my observations extend, on the side next the Maria, the parts of 

 the annuli opposite the Maria being more or less in their earlier state. 



The two undermentioned craters appear to be interesting examples of this class— 

 Fracastorius, situated on the border of the Mare Nectaris, and Hippalus on the 



