534 



Major J. Herschel. 



present, all reflections in order to proceed with my narrative. I 

 decided to flatten the wire slightly at the place where it passes the 

 split. Whether I did so insufiiciently or not at the right place it is 

 impossible now to say. The result was slipping, as before. I then 

 determined to have recourse to the loop — by which I mean giving to 

 the wire at the place where it is pinched a sharp bend back upon 

 itself. To do this the whole instrument had again to be dismembered. 

 1 cannot give an adequate idea of the anxiety attending a step of this 

 kind, in the case of an unfamiliar instrument of delicate construction. 

 My anxiety would have been greater perhaps, but hardly my care, had 

 I known, what I now learnt, that the gold wires were nearly as brittle 

 as untempered steel. In dismounting the major weight (without 

 which the lower attachment of the single wire could not be reached) 

 one of the double suspending wires snapped off short at the fastening. 

 I did not recognise the cause until on attempting to double back the 

 single wire to give it the crook it also snapped short. These mishaps 

 were experienced without much cost, for the new plan demanded but a 

 fifth of an inch of wire ; which the other end, in each case, could 

 easily spare. But before trying to bend it again I took the precaution 

 to anneal the end. At length, after a deal of trouble, I had succeeded 

 in fastening in this way both ends of the single wire, and of the 

 broken one of the pair. I ought to have done the same with the 

 other as well, but courage was wanting to go further in this direction 

 than accident had rendered necessary. The torsion on so long and 

 thin a wire (the pair are much thinner than the single one) turned 

 through only one quarter of a revolution, would probably be so slight 

 as not to exceed the holding power of the existing attachment. 



I should say here, that before putting the parts together again, 

 I weighed them, and measured the wire-lengths. The weighing is 

 elsewhere recounted. The lengths are 12"6 and 3'08 inches respec- 

 tively. I also took the opportunity to measure exactly the diameter 

 of the glass plunger, having reason to doubt the correctness of the 

 measurement assigned in Mr. Broun's description. The result justified 

 my suspicion. 



The end of all this is now at hand. In due time the instrument 

 was once more in a condition to recommence the adjustments. 

 Warned by previous experience I wasted no time over perfecting the 

 first ; but, noting the actual position of the lower mirror when at rest, 

 I turned it through two revolutions and allowed it to return. It 

 failed to reach its former place. 



I tightened the holding nuts to the utmost which the metal would 

 bear ; with no better result than to reduce the slipping, but not to 

 prevent it. The index test seemed to exonerate the upper holding, 

 but there was either slipping or straining — and that to a variable 

 extent — on every trial, whichever way the lower weight was turned. 



