367 
of Edinburgh, Session 1879-80. 
feet diameter is lowered over me into the water. There is a small 
hole in this cover, with glass over it, at which a light is held on the 
outside. Different quantities of marsh gas are then admitted under 
the cover, which I mix with the air by means of a fan. A reading 
is taken with the instrument, and at the same time a bottle of water 
is emptied and closed air-tight. The contents of the bottle are after- 
wards analysed ; and thus we obtain the true percentages correspond- 
ing to different readings of the scale. 
In this way I find it possible to measure the proportion of fire- 
damp to about J per cent. 
I have taken the instrument down several fiery mines, both in 
Yorkshire and Lanarkshire, and have found it most accurate and 
consistent in its indications. Messrs Merry & Cunninghame, after 
trying it, have adopted it. It is extremely portable, can be carried 
in a large coat-pocket, and is not likely to be injured, and causes no 
trouble. In fact, in this, its latest form, it seems to answer all 
requirements. Variations in the pressure of the air do not affect it. 
I ought to add that although choke-damp (i.e., carbonic acid gas) 
is not often found in company with fire-damp, yet even when this is 
the case, and in sufficient quantities to prevent the instrument from 
indicating the presence of fire-damp (choke-damp being as much 
heavier than common air as fire-damp is lighter), its presence pre- 
vents the fire-damp from being explosive ; and thus the indications 
of the instrument can in all cases be relied upon for indicating 
danger. 
4. On Comets. By Professor Tait. 
[Abstract.) 
The author commenced by stating that he had been led to make 
farther investigations, on the subject of his hypothesis as to the nature 
of comets, by some comparatively recent criticism to which that 
hypothesis had been subjected. Its main features had been published 
more than ten years ago in the “ Proceedings ” of the Society (May 
17, 1869) and in the first volume of “Nature.” Of course, if a 
critic completely misstates an hypothesis, he has no difficulty in 
refuting it ; so that to such writers the author does not attempt to 
reply. The other class of critics, including Mr Glaisher, and the late 
