Researches in British Mineralogy, 171 



of this transport and of its great intensity, the wave advances 

 more quickly, especially in the direction of the line of fire in the 

 first portions of its course than in the following ones. But this 

 acceleration quickly diminishes, and becomes almost insensible 

 when the wave traverses great distances. 



Two series of experiments may be distinguished. In the first 

 series, embracing eighteen reciprocal discharges from cannon, 

 each cannon was placed about 1280 metres from the membrane 

 which indicated the arrival of the wave. We find for the mean 

 velocity of propagation in calm dry air at 0° C, 



V' =331-37 metres. 



In the second series each cannon was distant about 2445 metres 

 from its corresponding membrane. This series embraces the 

 experiments during eleven days, consisting of 149 reciprocal 

 discharges of cannon, the conditions of temperature and wind 

 being very various. 



The general mean for the velocity in calm dry air at 0° C. is 



V' = 330-7 metres. 



This velocity is sensibly less than that of the first series. We 

 accordingly again find here that the velocity sensibly diminishes 

 as the path increases. 



In the second series the temperature varied from 1°'5 to 

 21°*8 C. I deduce from this that the correction for temperature 

 which is usually received is the true one. 



IX. These researches on the propagation of the wave in ga- 

 seous media were undertaken from the dynamical point of view 

 of the theory of heat. I believe that important deductions may 

 be made in this respect ; but the space at my disposal for this 

 extract prevents me from developing them. 



XX. Researches in British Mineralogy. 

 By David Forbes, F.R.S. fyc* 



II. 



Polytelite-f . 



LOCALITY : Tyddynglwadis silver-lead mine in the valley 

 of the river Mawddach, about six miles above Dolgelly, 

 North Wales. 



* Communicated b}' the Author. 



t Remarks having been made on the employment by me of "the rarely 

 used name Polytelite " instead of the uncouth German one Weissgiltigertz 

 or Weissgiiltigerz, I may explain here that, in adopting a name long ago 

 proposed by Glocker (and accepted by several other mineralogists) for such 

 argentiferous tetrahedrites as are specially characterized by so notably large 



