Photometrical Experiments. 45 
without the smallest alteration. Hente the dryness of air 
is indicated by the depression of temperature in a humid 
surface exposed to its action. But the air also communi- 
cates heat to water by pulsation: this would derange the 
results, were not the discharge of vapour subject to the same 
conditions as the emission “of heat, and in both cases the 
proximity of a vitreous or metallic surface produces effects 
entirely Similar, Nor is this mode of transmission by the 
play of alternate pulses confined to heat and moisture ; 
odours appear to be conveyed through the atmosphere by a 
similar azency. Jt is known that although the wind di- 
sperses widely odorous particles, yet their scent will pene- 
trate some distance against the current, and they may be 
concentrated by a tapering tube or reflector. This is proved 
by the action of the odour of ammonia on the colours of 
flowers, when exposed in the focus of a reflector. In this 
case the odorous substance, while dissolving i in the air, must 
have excited a sort of pulsatory impression like that of 
sound. The chief difference appears to be, that odours 
and moisture, consisting of matter sensibly ponderable, 
somewhat resemble wrecks floating on the waves, and are 
consequently not carried forward with the same accuracy, 
or to such a great distance, as heat, which possesses the in- 
herent and extreme subtility of lieht itself, 
The differential thermometer, by having one of its balls 
diaphanous, and the other coated with Indian ink or rather 
blown of deep black glass, will become a photometer, and 
indicate the comparative force of the light to which it is 
exposed. The rays falling on the clear ball pass unob- 
structed; but those striking the dark one are absorbed at its 
surface, where, assuming ‘2 latent form, they act as heat. 
This heat continues to accumulate till counteracted by an 
opposite dispersion, caused by the rise of temperature which 
the ball has acquired. JJence the space through which the 
coloured igi sinks in the stem will measure the moe 
mentary impressions of light, or its actual intensity. To 
prevent agitation, the instrument is covered with a bell- 
mie The photometer exhibits distinctly the progress of 
lumination from the morning ’*s dawn till noon, and thence 
its gradual decline tll evening. At Edinburgh the direct 
impression of the sun at noon during the summer solstice 
amounts to gO degrees ; but it regularly declines as his rays 
become more oblique, and at the altitude of 17° it is 
reduced to one half, at 3° above the horizon the whole 
effect exceeds not one degree, In winter the greatest 
force 
