12 Capt. Parry and Lieut. Foster 



of the lateral planes of this prism is 94° 6', and also that the 

 angle of the two oblique edges of the pyramid b ', which are in 

 a plane parallel to the modification //' is equal to 105° 37'. 

 Now considering that the plane of cleavage of Stilbite, which 

 is the face ,§•' of fig, 5, ought to be in position parallel to 

 the plane of cleavage of Heulandite, there are only two ways 

 in which it may be tried to derive the form fig. 5. from the 

 primitive adopted for Heulandite. The first is when the crystal 

 of Stilbite is placed, with respect to the primitive of Heulandite, 

 in the relative position represented by fig. 1. and 5; that is to 

 say, when the intersection of /^ • and g ' (fig. 5) is parallel to the 

 lateral edge of fig. 1. The second, when the crystal of Stilbite 

 is placed so, that the line of intersection of the planes /i', 

 g^ (fig. 5) is parallel to the intersection of b^ and g^ (fig. 2). 

 But in both cases it will be found that the faces b ' could only be 

 derived from fig. 1. by complicated intermediary decrements. 

 They might be made to correspond in the first case to the 

 planes of the two modifications the crystallographical signs 

 of which are ,, /- J. 2 in 



and in the second to the planes of the modifications whose 

 signs are r x , 6^ 



yd' b^^ g^ 9 j» {^1,2^ JTT /;T9 j. 



And even then the angles calculated from these indices would 

 not exactly correspond to the angles measured; and to obtain 

 a nearer equality between the results of observation and cal- 

 culation, higher numbers still should be used for the indices. 

 There can remain therefore no doubt respecting the incom- 

 patibility of the forms of Stilbite and Heulandite. 



III. Reply to Mr. Galbraith's Remarks oti the Experi7nents 

 for ascertaifiing the Velocity of Sound at Port Boisoen. By 

 Capt. W. E. Parry, R.N. F.R.S., and Lieut. H. Foster, 

 R.N. F.R.S. 



To Richard Taylor, Esq. 

 Dear Sir, H. M. S. Hecla, Deptford, Dec, 11, 1826. 



TN your valuable Magazine*, No, 341, is a communication 

 -*■ from Mr, Galbraith of Edinburgh, making some remarks 

 on our experiments for ascertaining the Velocity of Sound at 

 Port Bowen ; to which we should have replied earlier, had 

 not circumstances connected with our present engagements 

 interfered at the time of our becoming acquainted with the 

 remarks in question. 



In the first place, Mr. Galbraith seems to regret that in such 



* i^hil. Mag. vol. Ixviii. p. 214. 



a climate 



