TunnelU be made lencaththe Thames at G.avefend. 475'. 



great circumfpedion joined with fteadincfs : a little further the blue top is feen to be 

 more perfea and denfe, which if general through the place, all lights Ihcmld be extin- 

 guifhed, and the workmen be made to quit the place. Perhaps on advancing: a very little 

 further a light blue bead appears to circumfcribe the other dark blue, as if approaching to 

 name, and having a more than ordinary pointed fpire terminating like a white thread ; 

 and the danger becomes very great if to remain any time in this pofitlon. The next (lage 

 fliews the fprents to have taken place, which fprents are fimiiar to thofe produced by 

 fqueezing the oil from the rind of a lemon into the flame of a candle : at this period 

 fometimes has been obferved a fmall bufhy dark-coloured cloud hanging over the top of 

 the flame, flill more and more attrasSing the fire ; when in an indant perhaps after this laft 

 obfervation the whole appearance expands into flames, and one general conflagration takes 

 place. 1 he mifchief which enfues is in proportion to the accumulated quantity of this 

 pernicious vapour; which if great flies to the neareft aperture leading to the atmofphere, 

 being about eleven times lighter than common air, with a dreadful explofion and extraor- 

 dinary eruption. But in cafe tlie faid fire-damp or inflammable air be mixed with flylh, 

 or black damp, thefe appearances are fomewhat varied ; and by being mixed with a very 

 dark brown is in general more fafe. 



" Black damp or ftyth" (doubtlefs carbonic acid, or hydro-carbonate) " which arifes- 

 in mines is direflly the reverfe of inflammable air or fire-damp 9 the former extinguifhes 

 the flame of a candle as quickly as if put into water or any other fluid ; the other 

 caufes the candle to burn too faft. Ihis flvth or black damp prevails moft where 

 there is little or no inflammable air, and when- the due circulation of atmofpheric air is 

 iieglefted." , 



, It was of elTential confequence, by way of fliewing the praflicabiiity of the prefent urt-- 

 dertaking, that Mr. Dodd fliould imprefs the minds of his employers with the fad that 

 fuch tunnels had been made with fuccefs on a much larger fcale. For this purpofe he re- 

 lates various hillorical matters, befides thofe already mentioned, from which we may 

 notice, that the earliefl tunnel for inland navigation was executed by M. Riquet, to convey 

 the canal of Languedoc through a mountain near Bezieres. The firfl executed in this • 

 country was by the celebrated Brindley on the duke of Bridgewater's canal near Man- 

 chefter. The next is the famous tunnel of Hare Caftle Hill, in i)tafFordllnre, by the fame 

 engineer, which is 2880 yards long, and palTes through a variety of fttata, quickfands, &c. 

 more than feventy yards below the furface of the earth, and ferves as the receptacle for 

 part of the Grand 'Trunk Canal. The tunnel of Sapcrton is two miles and three quarters,- 

 long, and was carried through two miles of folid rock. Many other drifts or tunnels have 

 been made in this kingdom, through rocks and obftacles of various kinds, in a courfe of 

 time and at expence much lefs than would be apprehended by perfons unacquainted with- 

 •works of this nature. 



Mr. Dodd's report and eflimate on the projected dry tunnel under the river Tyne con- 

 tains illuitrations and argument of nearly the f;ime import. His pamphlet concludes with 

 a propofal for an inland canal from the Thames near Gravefcnd, to the Medway near 

 Stroud.; which, by a (hort courfe of about fix miles, would fave the circuitous and lefs 

 certain paflage of forty-feven. miles by the way of the Nore. The peculiar advantages of 

 this cut to the country at large, and to the government edablifhments on thefc rivers, as well; 

 zi the nature of the grouiKl itfelf,.are ftrongly and perfpjcuoufly Hated, 



