in the Island (^\St Michael. S13 
while others are porphyritic. This breccia is evidently of re- 
cent formation, and appears indeed to be actually forming at 
this time in those parts of the beds where it is soft, and the ce- 
ment gelatinous. The alteration which the rock has under- 
gone, in many places, where exposed to the steam and acid va- 
pours, is remarkable. The different substances composing it 
have lost theii* colours, and have now a pretty uniform degree 
of whiteness, the brecciated structure remaining. The frag- 
ments are soft, and in many places have acquired a distinctly 
argillaceous character. Some of the elevations composed of this 
breccia are upwards of thirty feet in height. Wherever cavi- 
ties occur in it they are lined with small stalactites, and bo- 
tryoidal concretions of pearl-sinter (fiorite of Thomson) and 
alum appears under the form of a delicate efflorescence, or in 
minute crystals. 
Besides the hot-springs already noticed, there are some others 
of less importance in different parts of the Valley of the Furnas, 
and bathing-houses have been erected in their vicinity. There 
are also many cold springs, the waters of which are abundant 
in carbonic acid and sulphuretted hydrogen gases, and they are 
strongly chalybeate. They occur in various parts of the plain, 
and some of them are so near the hot-springs, that the thumb 
may be placed in one of the former, the temperature of which 
is 70* or 80*, and the first finger of the same hand in one of the 
latter, at the temperature of 190® or 200®. The ground over 
which the water from the cold-spring passes, is covered with a 
thin coating of oxide of iron, and many of the loose stones have 
a beautiful metallic stain, which is sometimes iridescent. 
Art. XIV. — An Account of some Mistakes relating to Dr 
Bradley's Astronomical Observations^ and Harriot's MSS. 
By Dr Robertson, F. R. S. Savilian Professor of Astronomy 
in the University of Oxford. In a Letter to Dr Brewster. 
Dear Sir, 
- 1 - HF following statement of facts will, I trust, do .away ipis- 
takcs into which many of your scientific readers may Jiaye been 
