^^6 Ti/iry cf CJti Sukfrranfius IViiidt. 



rrate to a vfry little depth. From feveral known fafls, it docs not fccm probable that the 

 variations from thefe caufes would extend to the depth of three feet. Suppofc now the 

 external air to be cooled ten degrees ; the %vhole body Of air contained in the interftices of 

 the pottery, being lighter than that without, would afcend through the upper openings of 

 the mafs, in confequence of the preflure from the dcnfer air into the lower apertures. This 

 procefs would continue until the contents of the. edifice had acquired the common 

 temperature ; that is to fay (without attending to the capacities of pottery and of air for heat, 

 and the relative bulks of the interftices to the folid parts, but fuppofing thefe to be equal), it 

 would follow that a quantity of air near one thouf.ind times the bulk of the edifice mull 

 pafs through the interftices before the common temperature would be reftored. As the 

 winter advanced, the current would continue to flow in at the doors, or near the bafe of the 

 heap, and out from the fuperior parts. This would continue fo long as the temperature of 

 the external air continued to be colder than the internal parts of the heap ; and the velocity 

 would be greater the greater the difference between thefe temperatures. But when the 

 winter had reduced the whole mafs nearly to the freezing point, or perhaps below it, the 

 return of fpring, rarefying the outward air, would fulTer the internal cold denfe air to flow 

 out below. The warm air would neceflarily prcfs in above, become cooled, and flow out 

 again beneath in that flate of diminidied temperature. And in this cafe alfo the velocity of 

 the current would be greater the hotter the external air. 



It is eafy to fuppofe a prodigious variety of cafes. If the mafe be very large, the extreme 

 variations will be more confiderable, and the efl^ect more permanent and fleady. If the 

 fragments be large, the current will be fwifter, but the refrigeration lefs. j\nd if among 

 thefe fragments there be a conftant caufe of humidity, it may be inferred that this will 

 render the defcending current colder by evaporation than it would otherwife have been. 



I was led to mention the cathedral of St. Paul's, from having very frequently in hot weather 

 met a ftrong eool blaft ifluing out of the doors and vaults of that edifice, produced, as I fuppofe, 

 by the refrigeration of the air agaioft the ponderous mafles of mafonry within its area. As 

 the probable caufe did not occur to me till the prefent inveftigation offered itfelf, I have not 

 examined all the circumftances of this laft fa£l. Currents of this nature are however very 

 common. The well-known experiment of holding a candle firft at the top and then at the 

 bottom of a door, is of this kind. If the air within the room, or the walls, be warmer than 

 the external air, the flame will be blown outwards at the top, and inwards at the bottom 

 of the door ; but the contrary will happen if the walls of the room or its contents be colder. 

 In this cafe, the lower current refembles the cooling blaft of the caves. There is no doubt, 

 if two holes were bored in the door of a celbr in fummer, the one near the top and the 

 olher near the bottom, that the upper hole would draw in the air, and the other emit a cold 

 ftream, until tlie walls had acquired the heat of the external air. 



X. 



The Comtuftion of Pkofphcrtir in the Vacuum of the Mr Pump. By DR. MaRTINVS VaIJ 



Madvm*. 



X. JlIaVING propofed in December 1794. to eithibit in one of the ledures on the Tey- 



leriau fcundation the combuftion of phofphorus in oxygene, and its combination with that 



• Aniialcsdc Cljmit, XXI. it3. 



f ' fubftance 



