ON THE CONSTRUCTION OP LARGE REFLECTING TELESCOPES. 79 



from the papillae ; they are then separated by a lateral motion, which ex- 

 tends the viscid matter into fine filaments connecting the papillae ; on these 

 filaments the current impinges, drawing them out to a length which is regu- 

 lated by the will of the animal ; and on the mammulae being again brought 

 together the filaments coalesce and form a compound line. 



Many intelligent naturalists entertain the opinion that spiders can forcibly 

 propel or dart out lines from their spinners ; but when placed on twigs set 

 upright in glass vessels with perpendicular sides containing a quantity of 

 water sufiicient to immerse their bases completely, all the elforts they make 

 to effect an escape uniformly prove unavailing in a still atmosphere. How- 

 ever, should the individuals thus insulated be exposed to a current of air 

 either naturally or artificially produced, they immediately turn the abdomen 

 in the direction of the breeze, and emit from the spinners a little of their 

 viscid secretion, which being carried out in a line by the current becomes 

 connected with some object in the vicinity, and affords them the means of 

 regaining their liberty. If due precaution be used in conducting this experi- 

 ment, it clearly demonstrates that spiders are utterly incapable of darting 

 lines from their spinners, as they cannot possibly escape from their confine- 

 ment on the twigs in situations where the air is undisturbed, but in the agi- 

 tated atmosphere of an inhabited room they accomplish their object without 

 difficulty. Similar means are frequently employed by spiders in their natural 

 haunts for the purposes of changing their situation and fixing the foundations 

 of their snares. 



The webs named gossamer are composed of lines spun by spidei-s, which 

 on being brought into contact by the mechanical action of gentle airs adhere 

 together, till by continual additions they are accumulated into irregular white 

 flakes and masses of considerable magnitude. Occasionally spiders may be 

 found on gossamer-webs after an ascending current of rarefied air has sepa- 

 rated them from the objects to which they were attached, and has raised 

 them into the atmosphere ; but as they never make use of them intentionally 

 in the performance of their aeronautic expeditions, it must always be regarded 

 as a fortuitous circumstance *. 



On the Construction of Large Reflecting Telescopes. 

 By the Earl op Rosse. 



The Council having intimated their opinion that some account of the ex- 

 periments in which I have been engaged on the reflecting telescope would not 

 be altogether devoid of interest, 1 will endeavour to describe as briefly as pos- 

 sible the manner in which I have attempted to accomplish the object in view, 

 and the principal results obtained. 



Having concluded that upon the whole there was a better prospect of 

 obtaining by reflexion rather than by refraction the power which would be 

 required for making any effectual progress in the re-examination of the 

 nebulae, the first experiments were undertaken in the hope of obviating the 

 difficulties which had previously prevented the application of the brilliant 

 alloy, which may be formed of tin and copper in proper proportions, to the 

 construction ol" large instruments. The manner in which the difficulty had 

 been met was by adding an excessive proportion of copper to the alloy, but 

 the mirror was no longer susceptible of a durable polish, and when used its 

 powers declined rapidly. 



* Transactions of the Linnxan Society, vol. xv.p. 449-459. Researches in ZooIogy,p. 229-252. 



