266 Scientific Intelligence. 
fasten it to the.ceiling. It is essential to success that the mercury 
should not be so heavy as os injure the elasticity of the strap, or sub- 
ject it to its maximum of tension. Messrs. Seguin and Mauvais have 
thus secured good results an the caoutchouc band ; ; they have found 
that in order to extinguish the vibvatious indicated, prt by trac- 
tion is preferable to elasticity by pressure. The solut 0 this diffi- 
culty will be welcomed by astronomers, and capella by M. Airy, 
who after having ee a thick bed of sand in one of the halls of 
the Greenwich Obse rvatory, for his Pina horizon, still found that it 
had not the steadiness required by his zenith reflection telescope, aa 
who has extended his Segara ~ nearly all the instruments used for 
measuring the intensity of cu 
pretz has treated this yearn profoundly by a series of experi- 
ments which aw limits of this communication will not allow me to Te 
port. The fundamental fact established is that the tangents of the de- 
viations are not necessari ily proportional to the intensity of the current, 
d that they can be considered proportional only when the great circle 
in which the current passes has a diameter of a meter, and the le! 
of the needle is not under 3 centimeter 
Such large needles will in fact have little sensitiveness ; but they may 
be made as sensitive as small needles, by substituting, for the plate of 
the great circle, four large wires, 5 to 8 millimetres in section, insulated 
from one another by being wound with silk. 
The tangents of the deviations of this instrument por ater 
sity “a ayn currents, and we have thus a true proport heometer 
replace the four large wires by a bundle of twelve to twenty 
wires ‘of smaller size, (for example, three to four ee wou 
have proportional rheoscopes, sufficiently sensitive 
intensity of the feeblest currents. In order that the refer 
ded circle should correspond with the precision of the spar 
necessary that the circle should be about 30 centimeters in dia 
These rheometers and feonanias may also be used for 
ordinary rheometers and rheosc and 
The formula used by Despretz for the pr ere of his results, 
which accords with experience, is not ie nary formula, 
Te 
in which T represents the intensity of alba magnetism, 
more complex formula, 
2 
I=(1+3e?) tang Give —_ sin 20 
which is founded on the theory of Presto In this formula, I repre 
but the 
of the needle eae the radius of the iaie of the re the Acade- 
its obj 
