36 Account if Varieties of Sulphate of Stronltan. 



(both which events are continually prefcntingthemfelves), the rtrokes of his wings maybe ob- 

 ferved from the refledions of the light, which exhibit a row of bright fpecics or ftars. When 

 the fly moves at the rate of about five feet per fecond, the intervals are about ten in the inch ; 

 but in very quick flight, to avoid danger, he can advance near three quarters of an inch per 

 ftroke. Now 5 X 12 x 1-0=600 ftrokes per fecond, in the fit ft cafe. I could not e'ftimate 

 the velocity in the fecond cafe ; but fuppofe it might be fix or feven times as much, or about 

 30 feet per fecond*. No found was produced in either cafe ; whence, I fuppofe, the found 

 of the wing, when the fly is caught, or the fhrill tone of an gnat when it fticks itfelf in the 

 melted tallow of a candle, are produced by much flronger and quicker a6lions. 



If you fhould tliink thefe obfervations may be acceptable to your readers, they are at your 

 fcrvice ; but ^f not, excufe the trouble 1 give you, and fupprefs them. 



I remain, Sir, 

 ■ - Your obliged reader, 



S.R. 



IX. 



An Account of fever al Veins of Sulphate of Strontlan or Strontites, found in the NetghbourhoeJ^ 

 of Brijiol, with an Analyfis of the different Varieties. By WJLLIAM. ClayfI£LI>\. 



i H E firfl fpecimen of fulphate of ftrontlan was (hewn to me by Mr. Tobin, about three 

 years fince. At that time it was generally believed to be merely a variety of fulphate of barites. 

 It had been found at Redland, a (hort time before, in a vein of confiderable thicknefs. The 

 greater part of this vein has received a red tinge from the iron ftone on which it lies, and ex- 

 hibits but flight traces of any regular cryftallization. In fome few fituations, however, it is 

 entirely free from colour, and appears to be compofed of a confufed mafs of bevilled tables, 

 loofely adhering together. Its fpecific gravity varies from 3,5 1, to 3,87. Walking along the 

 beach at Auft-paflage, in June, 1797, I met with a fimilar fubftance, and foon difcovered 

 feveral detached veins in different parts of the cliff. The ftrata in which thefe veins are found 

 are nearly horizontal, confiftingoflimeftoneof different degrees of hardnefs, and argillaceous fand- 

 ftone, intermixed with clay and gypfum. The whole cliff, as well as the furrounding country, have 

 evidently been produced by aqueous depofition, fince which period, the level of the water in the main 

 channel having been confiderably lowered, the Severn current has acquired fuflicient force to 

 deepen the bed of the river by plowing up the ftrata which had been previoufly formed. 



A fecond depofition of the foil towards the mouth of the river, by forming diagonal fiffurcs 

 in the cliff, has occafioned an inequality of five or fix feet in the level of the flrata. 



Thefe fiffures are moftly filled up with veins of ftrontian, from three to twelve inches in 

 thicknefs, confifiing of an affemblagc of femi-tranfparent cryfl^als, flanked up «n each fide by a 

 tbin layer of a fibrous fradture ; both of a delicately blue tinge.. This laft variety was obferved 



' • A racc-horfe, at firft darting, will clear 90 feet per fecond, making about four, or, perhaps, five leap* 

 each fecond. 



■(• From " ContributioDj to phyfical and medical Knowledge, &c. &c." collected by Thomas Bcddoes, M. D. 

 See the next article. 



about 



