93 
and the Mode of its Communication. 
a coating of very fine, transparent, and colourless spirit varnish; 
and, after this coating of varnish had become quite dry and hard, 
I repeated the experiment above-mentioned ; and, finding that 
this covering, like that of glue, expedited the cooling of the 
instrument, I first added a second coating of the varnish, and 
repeated the experiment again, and then added two coatings 
more, making jour in all. Finding that the cooling of the in- 
strument was more and more rapid, as the thickness of the 
varnish was increased, I now added four coatings more, making 
eight coatings in the whole, giving time for each new coating 
to dry thoroughly, before the next was applied ; but I found, 
on repeating the experiment with this thick covering of varnish, 
that I had passed the limit of thickness which produced the 
greatest effect. 
In order that the results of these experiments, with coatings 
of different thicknesses of spirit varnish, may be seen at one 
view, I shall here place them all together; and I shall place by 
the side of each, the result of the standard experiment, which 
was made at the same time, with the instrument No, 1, the sides 
of which were naked . 
Time employed in cooling through 
the given interval of io degrees. 
Instrument Instrument 
No. i. No. 2, 
varnished. naked. 
Exper. No. 4. 1 coating of varnish - 42 min. -» 53^- min. 
Exper. No. 5. 2 coatings - - - 35% - - S 5 i 
Exper. No. 6 . 4 coatings - 30^ - - 
Exper. No. 7. 8 coatings - - - 34J- - - 
Exper. No. 8. Desirous of finding out what effect colour 
would produce, I now painted the sides of the instrument No. 2 
black, with lamp-black mixed up with size, (this paint being laid 
