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Scientific Intelligence— Astronomy. 357 
that is, till the temperature rose sufficiently. Or, as sulphocyanide 
of potassium was the test employed, the test may have contained a 
trace of iron. I have tested with galls after neutralising with am- 
monia, and the other tests usually employed, and could not detect 
any compound of iron in the acid. 
SCIENTIFIC INTELLIGENCE. 
ASTRONOMY. 
1. On the Extinction of Light in the Atmosphere. By W.S. 
Jacob, Esq., H.E.I.C. Astronomer, Madras. Communicated by 
Professor Piazzi Smyth.—In a letter dated Madras, November 
1849, Captain Jacob says, “ I have been much interested in read- 
ing, lately, Professor Forbes’s paper in the Philosophical Trans- 
actions, 1842, Part 2, on the Extinction of Light and Heat in 
the Atmosphere.” As his results agree very closely with those 
of my experience on the Trigonometrical Survey of India, and - 
which, though not founded on any precise measures, being still 
the conclusions of some years’ experience, are perhaps worth noticing, 
particularly when they agree with the results of more exact measures, 
On commencing work with heliotropes in 1837, I soon found that 
for long distances it was necessary to enlarge the apertures more 
than in the simple ratio of the distance (though such was Colonel 
Everest’s practice) ; and before the end of the first season, I had 
formed a scale of apertures for corresponding distances, which after- 
wards needed very little alteration, but when finally corrected by 
subsequent years’ observation, stood as follows :— 
Aperture. Maximum Distance. Maximum Distance 
inches. Miles. without Absorption, 
0°5 15 15 
1:0 23 30 
2:0 33 60 
4:0 45 120 
8:0 60 240 
Our heliotropes were circular glass mirrors, 8 inches in diameter; 
and for the smaller apertures, diaphragms were used between the 
heliotropes and the observer. At the distances stated the light was 
just visible to the naked eye in clear weather, and when seen over a 
valley: if the ray grazed near the surface, the light was much re- 
duced. On one occasion I employed a heliotrope at 6} miles, and 
used an aperture of } of an inch, and found it rather brighter than 
usual, so that probably 6} or 7 miles would be the normal distance 
for that size, 
