of Siphon Barometers. 209 
mean atmospheric pressure, and adopting for the coefficients of 
cubic expansion of mercury and glass for one degree Centigrade 
the values 
m='0001803, g,='0000258, 
we obtain 
[= 2°463 inches. 
This indicates a perfectly practicable arrangement. To ren- 
der the temperature correction insensible at mean atmospheric 
pressures when the siphon tube has a uniform bore, we must 
put so much mercury into the tube, that, when the pressure is 
29°5 inches, there shall be a column of about 2°5 inches of mer- 
cury in the open leg. The temperature corrections throughout 
all ordinary fluctuations of atmospheric pressure for the lower 
surface of the mercury will then be extremely small, as will be 
seen by the following Table :— 
Column of mercury Displacement of | Displacement of 
Atmospheric in open leg of ~ | Upper surface for a| lower surface of 
pressure siphon difference of tempe-| mercury for differ- 
(h). rature Az in Centi-| enceof temp. Aé 
grade degrees. in Cent. degrees. 
31 inches. 1-713 inch. -+'00545 Az —°00014Aé 
295 ,, 2-463 ,, +-00531 At -00000A¢ 
Per ser, 3-213 ,, -L-00518A¢ +-00014A¢ 
It may be well to observe that the numbers in the last column 
of the Table show that if the barometer to which they refer 
were heated, as in the experiments already described, and if the 
atmospheric pressure were much greater than 29°5 inches, we 
should have the result obtained by M. Negretti—a depression 
of the mercury in the short leg of the siphon; while if the pres- 
sure were nearly 29:5, there would be no sensible change of 
level, as observed by Mr. Bryson. 
I need scarcely remind the reader, in conclusion, that the for- 
mule I have investigated are intended to be employed when only 
the upper or only the lower surface of the mercury is observed. 
When Joth surfaces are observed, as in the Bunten barometer, 
we have simply to apply the ordinary and well-understood cor- 
rection, due to the expansion of mercury by heat. 
United College, St. Andrews, 
February 16, 1861. 
Phil. Mag, S. 4. Vol. 21, No, 189. March 1861 P 
