Mr. N. S. Maskelyne on Ludlamite. 525 



block is insulated from the spring, which carries it, but is 

 connected with the point B. The interval elapsing between 

 the breaking contact at m and the making at n is, and must 

 be, utterly inappreciable. For the shortest practicable inter- 

 val is ^sufficient to allow the currents to adjust themselves, 

 unless a and b are extravagantly large resistances ; and if it 

 is not exceedingly short, disturbances will occur due to varia- 

 tions in the battery. 



The diagrams purposely show the galvanometer, and not the 

 condenser, connected with the breaking-key ?i, because leak- 

 age of the condenser is sure to occur to some extent when its 

 circuit is broken, and, in order that this may not in the least 

 signify, one terminal of the galvanometer must be insulated. 



University College, London. 



LXX. Additional Note on Ludlamite. 

 By K Story Maskelyne, F.R.S.* 



THE column of calculated angles incorporated in my notice 

 of Ludlamite is vitiated by an error traceable to the 

 erroneous reading of a figure in the calculation of one of the 

 fundamental angles ; and a consequent revision, at somewhat 

 greater leisure, of the relative weights to be attached to the 

 measurements of the different angles on the various crystals, 

 has enabled me to offer a closer approximation to the true ele- 

 ments of Ludlamite than I was able to give at the time the 

 measurements were published. The subjoined data therefore 

 present a better theoretical representation of the crystallo- 

 graphy of Ludlamite than that published in the Philosophical 

 Magazine of January last. 



System oblique, the elements of the crystal being 



101.1 = 42° 46', 1 1 1 . 1 = 36° 37', 

 101.001=36° 41'; 

 otherwise, a : b : c = l : 0'4439 : 0*8798. 



77 = 100° 33'; I e. the axial angle ZX = 79° 27'. 



r-ac =79 27 

 cd =45 53 

 da' = 54 40 

 at =26 491 



UV = 31 561 



Tarn =65 41 J 

 [mm' = 48 37 





"cm = 85 40| 

 cq =68 31 

 cp =61 251 



_cr =44 361 



r crp=64 1 

 al =85 12f 

 Iq =22 30| 



Iq of = 72 171 



[qq r =63 28| 

 L^g =58 15f 

 pn =53 23 



[ 



< 



h q = 60 58J 

 &m = 110 27^ 

 :l = 62 50 



Communicated bv tlie Author. 



