140 
THE VOYAGE OF H.M.S. CHALLENGER. 
!: v. the Correcting factor l>y one reading. Tt consisted of a kind of very short but 
v. C v-Lussac burette, provided with a very narrow long side tube. The wide tube 
: : 1 . \ ing been charged with a little water (and air) was hermetically closed above and 
. and suspended in the bath of the measurer. Obviously the factor for reduction 
• ■ - v 15° < and 7G0 mm. is a function of the difference of level in the two tubes, and it 
i <\ for a given charge of water to standardise the apparatus by preliminary experi- 
m« nt>. and enter in a table the factors for the several values of the difference of level. 
Tin' apparatus was tested by plunging it into a water-bath of known temperature, and 
<■ tin -ting the open end of the narrow tube with an artificial atmosphere, whose excess 
• a- >1 licit of pressure was measured by a water-manometer. It w r as found to give correct 
indications. It was more delicate than our barometer and thermometer used conjointly; 
\ t wo hardly ever used it in earnest, because we soon found that it took less trouble to 
’ do an occasional calculation than to keep the regulator in order. Perhaps the little 
nt may be useful to others engaged in gasometric work; hence I have described 
it here. 
The calibration of the measurer is effected by means of an exact balance. We used a 
1 6 inch Oertling. ” For this purpose the measurer is provided temporarily with a 
-top-cock below, so that it can be used like a Mohr’s burette. It is then filled with 
nu n ury. the side tube to overflowing, the graduated tube to a little beyond the stop-cock. 
I i - * india-ruhbi-r tube below, which established communication with the reservoir, is then 
r< mov.-d, the stop-cock at the head of the side tube closed, and next mercury run out 
* t burette until the metal stands just at the inside end of the stop-cock. Everything 
i ,( W i ]■ idy f"r the calibration. As a necessary first step, mercury is first run out very 
cautiously, bo far as t<> bring the metal i" the point where the capillary appendage joins 
"it I d ruptly) to the burette, and weighed, and then successive convenient instalments of 
1 ■ run out, and at each step the total mercury discharged so far is weighed, and 
t’ r , iding of the meniscus in the tube taken by means of the telescope. The telescope 
nd rt nd I used for all mv gasometric work came from Mr. ( a sella, and I found it in 
■ !■ -j" ct i most excellent instrument, both optically and mechanically. 
I hiring 11 this work the burette stands in the water-bath, and is thus kept at a 
j ’ !!y constant temperature. This at first sight may appear to be an unnecessary 
■ ■ ; but ex jM-riencc -bowed (in confirmation of what mere thermometer-readings 
ei might have revealed beforehand) that, in an ordinary room of fluctuating 
'nr , a degr.-c of precision which does justice to an accurate millimetre scale as 
r ad by a good telescope, cannot otherwise be attained. 
I ' ' btcfl to I’rofi -or I ait for having kindly placed at my disposal an excellent 
i engine, made by Itinnchi of 1'aris, and thus enabled me to provide all my 
gasom* trie apparatus with faultless millimetre-scales. 
‘ ' ,1 of tie calibration table, one grm. of mercury was taken as re- 
