197 



A phenomenon, not less interesting and astonishing, is the ap* 

 pearance of floating islands on this lake. These rise, at uncertain 

 periods, from the bottom of the lake : sometimes one, and some- 

 times two, have arisen in a year; and, at other times, several years 

 may elapse without their appearance. Their time of remaining at the 

 surface is also very uncertain ; some sinking again, within twenty- 

 four hours ; whilst others remain for six weeks, two months, or even 

 longer, before they return to the bottom of the lake ; which they 

 in general do without having suffered any change in their form : 

 but sometimes they burst, and are so widely rent, that boats can 

 pass between the separated parts. One rose in 1798, which was 

 one hundred and eighty yards long, and fifty yards wide. They 

 sometimes show above a foot high of land, out of the water; and 

 some have been measured which have been seven yards in thickness. 

 The upper part of these masses, for about two feet in depth, is 

 common mud ; beneath this it becomes less compact, and the re- 

 maining part is found to be formed of decayed leaves and roots of 

 plants and trees. 



A pole being passed three or four feet into them, a considerable 

 quantity of air rushes out on its being withdrawn : this air smells 

 like gunpowder ; and it is said that, if collected in a bottle, it will 

 take fire if exposed to the flame of a candle*. 



The most prominent circumstances in the foregoing relation are, 

 the matter of which these islands are chiefly composed; the gaseous 

 matter which is so plentifully liberated, from an opening made into 

 its substance by means of a pole ; and the extraordinary impulse, 

 and agitation by which the water of the lake is affected. These 

 several circumstances we will therefore subject to a careful and 

 impartial examination. 



All, except about two feet of the upper part, of these islands, 

 appears to have been formed of the decayed leaves and roots of 



* Phil. Mag. No. XLII. 



