ON EIGHT BALLOON ASCENTS IN 1862. 377 
horizontal vibration; hermetically sealed glass tubes from which air had 
been exhausted ; ozone papers ; and an electrometer lent by Prof. W. Thomson 
of Glasgow. 
Barometers.—The mercurial barometer employed in all the ascents was 
a Gay-Lussac’s siphon barometer by Mr. P. Adie, and is one of those 
used by Mr. Welsh in the year 1852 in his experiments. The inner 
diameter of its tube is 0:25 inch. ‘The graduations were made on a brass 
scale, from its middle point upwards and downwards ; each division was about 
0:05 inch in length, representing twice that value, so that an observation 
of either the lower or upper surface of the mercury would give the approxi- 
mate length of the column of mercury. 
The readings of the upper end were alone taken, and the corrections appli- 
cable to this end have been applied to all observations. 
The barometer was furnished with its own thermometer, whose bulb was 
_ immersed in a tube of mercury of the same diameter as that of the barometer. 
This instrument sometimes read more than 20° in excess of that of the 
sensitive air-thermometer. 
The aneroid barometers were made by Messrs. Negretti and Zambra; one 
was graduated to 13 inches, and the other to 5 inches—the latter instrument 
having been used in the ascents on August 18 and September 5, and the 
former on July 17. In consequence of a difference of reading between the 
aneroid and mercurial barometers on July 17 (and as both instruments were 
broken, it was impossible to say which was in error), and as the correctness 
of the siphon barometer at low readings is dependent upon the evenness of 
the tube, another barometer was used in addition on September 5, made by 
and at the suggestion of Messrs. Negretti and Zambra, as follows :— 
A tube 6 feet in length was filled with mercury and boiled throughout 
its whole length; a glass cistern was blown on the bottom of the tube, and 
bent upwards in the form of a siphon; a stopcock was placed between the 
tube and cistern, and whilst the mercury filled the entire tube, a mark was 
made on the cistern, at the level of the mercury in it, for zero; the stop- 
cock was then gradually opened, and the mercury allowed to descend one or 
more inches. The rise which consequently took place in the cistern was 
carefully marked on the same side as “0” (zero); the stopcock was again 
opened and the same operation was repeated until 30 inches of mercury had 
left the upper part of the tube, and the successive levels of the mercury in the 
cistern had been accurately marked. 
In finally making the barometer, the upper portion only of the tube was 
used; the cistern which had been at the end of the lower portion was 
removed and joined on the upper; and in graduating the scale of the 
barometer, the rise which took place in the cistern at every. inch was 
deducted, and the scale reduced in its entire length, by the exact amount 
of the rise of the mercury in the cistern. This instrument was therefore 
probably as accurate at low readings as at high. 
Dry- and Wet-Bulb Thermometers—Two pairs of dry- and wet-bulb 
thermometers were employed; one pair as ordinarily used, their bulbs being 
protected from the direct rays of the sun by a double highly polished silver 
shade, in the form of a frustum of a cone, open at top and bottom. A 
cistern was fixed near to them, from which water was conveyed to the wet- 
bulb thermometer. 
The bulbs of the second pair of dry- and wet-bulb thermometers were 
enclosed in two silver tubes placed side by side, and connected together by 
a cross tube joining their upper ends, and over both were placed double 
1862, Zc 
