452 
THOMSON ON THE FORM ACTION OF SHLPHURIC ACID. 
azimuth circle. In these the repeating prin- 
ciple so much recommended in Borda’s, is 
dispensed with for the purpose of securing 
stability and permanency of adjustment, 
which are the main desiderata in the other. 
Though the same principle of repetition 
cannot be practised by these instruments as 
in that of Borda, yet the observations may be 
repeated near the meridian with success, in 
which the constant error arising from the 
imperfection of the repeated damping on 
Borda’s plan, is thereby avoided, while by 
means of three verniers carefully made, com- 
bined with the motion of the celestial body in 
zenith distance during the time of repetition, 
the errors of division and pointing will, if not 
entirely destroyed, be greatly diminished — a 
proposition suiiported by uniform experience. 
In this country the use of the great mural 
circle permanently fixed in the meridian is 
generally adhered to, and by means of its 
size, the power of its telescope, and the num- 
ber of its reading microscopes, its errors are 
supposed to be almost entirely destroyed, 
though the principle of repetition be aban- 
doned. Thus by the introduction of one 
advantage, another is lost instead of attempt- 
ing a union of both. The smaller circles 
possessing the property of repeating the ob- 
servations, may, therefore, approach very 
nearly the precision of the larger, as has been 
proved in the measurement of the French arc 
of the meridian compared with the British 
trigonometrical survey. It is indeed difficult 
to say, whether the final results of the one 
or the other possess the superiority, though 
the former was executed chiefly with Borda’s 
repeating circle of about 16 inches diameter, 
or 8 inches radius both with regard to astro- 
nomical and geodetical observations, while 
the latter had the benefit of a zenith sector 
of 8 feet radius, and a theodolite of 3 feet in 
diameter, both without the principle of repe- 
tition adopted by Borda. Hence, it may in 
general be concluded, that the principle of 
repetition employed in one class of instru- 
ments was nearly equivalent in securing accu- 
racy of results to the advantage of large size, 
and the superior power of the telescopes in 
the other. Hence, we may also infer, that 
one of Mr Troughton’s objections to the 
repeating circle, namely, that wdien the in- 
strument has a telescope of small power the 
observations are charged with errors of vision 
which the repeating principle will not cure, is 
not borne out by experience. Indeed we can- 
not comprehend the notion why the errors of 
vision as well as of division according to the 
usual doctrine of probabiltties are, if not de- 
stroyed, at least greatly diminished, by the 
principle of repetition. MM. Lenoir and For- 
tin have lately improved the movements of 
the repeating circle in some respects according 
to Puissant, and Mr. Troughton has given 
some of its parts abetter position for dimi- 
nishing friction and insuring accuracy of 
motion, though on the whole it is still com- 
plex in its construction, and, so long as its 
peculiar repeating principle is retained, it 
cannot be much simplified. The late M. 
Reicheubach,of Munich, constructed repeating 
circles, which for some time have enjoyed 
great reputation, chiefly on account of the 
goodness of the tangent and damping screws, 
and the accuracy of the division. The superior 
telescope is also attached to a circle turning 
with ease and precision within the graduated 
circle and having their faces both in the same 
plane. 
On the recommendation of Laplace this 
artist const! acted a large repeating circle of 
this description for the Royal Observatory at 
Paris. Whatever properties it may possess 
yet it has been thought advisable to have also 
a large mural circle on the principles of Mr. 
rroughlon constructed by Fortin, which the 
French express a liope will contribute greatly 
to the advancement of astronomy. From 
these circumstances it seems to be admitted 
that the powers of the repeating circle have 
been overrated, and that the advantages deriv- 
ed from the repeating principle aie in a gieat 
degree counterbalanced by the delects of 
Its consliuction. May we, thereiore, irjfer 
that the smaller class of portable circles of 
the constructions of Troughton and Kater, 
which admit of repeating the observations 
near] the meridian a sufficient number of times 
to insure accuracy, are, from the compactness 
of their structure, their stability, and ac- 
curacy of motion, superior to the repeating 
circle. Of al! these circles Kater’s is the 
cheapest and susceptible of great accuracy 
when not too small. The size 1 would ven- 
ture to recommend would be about 6 or 8 
inches in diameter with telescopes magnifying 
20 or 30 times and the three verniers each 
reading 10 . To those who are willing and 
able to afford the expense, one of Troughton’s 
altitude and azimuth circles of 10 or 12 inches 
in diameter would prove an excellent instru- 
ment, liiough for travellers it would be rather 
to,) heavy. lu th >t case Katei’s would be 
a good substitute, and its efficiency will be 
snov\ trill the following observations. 
{To he continued.) 
ON THE FORMATION OF SULPHURIC 
ACID. 
By Thomas Thomson, M. D. F. R, S- 
L. and £. &c. 
Regius Professor of Chemistry in the University 
of Glasgouj. 
It is well known that sulphuric acid is ma- 
nufactured in this country by the combustion 
of sulphur. The sulphurous acid formed is 
passed into large leaden chambers, where it 
comes in contact with nitric acid and a small 
quantity of water ; the fumes of the nitric 
acid being sent into the leaden chamber at 
the same time with the sulphurous acid. 
Now, whenever any sloping part occurs in the 
leaden chamber at some height above the floor 
which is covered with water, there is a de- 
posit of a white saline matter. 
