82 On Artificial Cobwebs for Micrometers. 



is required to reduce the India rubber to the consistence neces- 

 saiy to form cobwebs, in order to allow impurities to subside to 

 the bottom of the phial, and to insure the complete solution of 

 the caoutchouc, for if any particles of it are left untouched, 

 smooth even cobwebs will not be obtained*. This solution 

 must be kept about the person till a good deal of the turpentine 

 has evaporated, to aid which process it will be advisable to have 

 the phial merely closed by a piece of paper tied over it : the 

 older it is the better. If any heat is employed to effect the 

 solution, exceeding that of the human body, the consequence 

 will be, that the caoutchouc will undergo a change, and will 

 never dry or return its original state. AVhen the India rubber 

 in the phial has assumed a viscid consistence, like that of bird- 

 lime, it is in a fit state for making cobwebs : the precise con- 

 dition in which it is most fit for that purpose may be known 

 by experiment, and by its sticking two other pieces of India 

 rubber together in such a manner, that they cannot be sepa- 

 rated when the interposed cement is dry. 



The making of the cobwebs is the most simple affair imagin- 

 able ; all that is necessary is to take a small quantity of the 

 solution on the point of a bit of wood, and to stick it to a frame, 

 producing the viscid thread, which will proceed from it to the 

 opposite side : in this way any quantity may be made. This 



* There will always be the same diiRculty in obtaining truly parallel 

 and even threads from the caoutchouc as there is from common cob- 

 webs. I have one of Messrs. Troughton and Sims's best micrometers by 

 me at this moment ; and the cobwebs, though, of course, picked and 

 abundantly even enough for practical purposes, are certainly far from 

 being of the same diameter throughout. I suspect, from microscopical 

 examination, that a common cobweb is a flat thread like a piece 6i tape, 

 not a round one, so that, if it is at all twisted, it must present different 

 apparent diameters to the eye. The best way of getting parallel threads 

 with the India rubber paste, appears to me to be to let it stand for a con- 

 siderable time without any agitation, or stirring up whatever, and when 

 the instrument is inserted to draw out a thread, to disturb the matter as 

 little as possible, touching only its surface ; the more the thread is drawn 

 out the more even and true it becomes. It may not be irrelevant to 

 mention, that in order to give the fibres their utmost degree of strength, 

 tliey should be put on in the lowest degree of tension possible ; in draw- 

 ing them out on the frame, therefore, let the instrument recoil as much 

 as it will before the end of the thread is fastened down, that at least the 

 caoutchouc may not be stretched till its ultimate attachment to the 

 micrometer is completed. Rectified white naphtha may be tried instead 

 of the turpentine. 



