Dec. 8, 1881] 
NATORE 
127 
THE PRESSURE ERRORS OF THE 
“ CHALLENGER” THERMOMETERS} 
Ville, 
XI. Accurate Measurement of Great Pressures. 
T will be obvious from what has been said, especially as regards 
the old apparatus which was carried about in the Challenger, 
that one of the most essential requisites of the whole investigation 
was the accurate measurement of pressure. All the ordinary forms 
of pressure-gauge were found to be untrustworthy. It was neces- 
sary that in all cases the pressure should be measured with cer- 
tainty to about 1 per cent. No attempt was made to secure any 
greater degree of accuracy, as the indications of the thermo- 
meters themselves could not in any case be trusted to less than 
o*1 Fahr, 
The basis on which, after a great many trials, I finally founded 
my determination of pressures, was Amagat’s? remarkable 
measurements of the volume of air and other gases at high 
pressures. Amagat’s data were obtained in the most direct and 
satisfactory manner, inasmuch as he measured his pressures by 
means of an actual column of mercury extending sometimes to 
300 metres, and more. All other means of measuring pressure 
are as it were valueless in comparison with this. We know by 
these experiments the compressibility of nitrogen, and of air, up 
to pressures of at least two anda half tons weight per square 
inch, with almost all desirable accuracy. 
All that was necessary therefore in order to determine the 
pressures in the operating cylinder, and thus to calibrate the 
gauges employed, was to compress once for all a quantity of air, 
measure the volume to which it was compressed and the cor- 
responding indications of the gauges, and then by the help of 
Amagat’s tables compute the pressure actually attained. The 
apparatus I employed for this purpose is | gured in section in the 
diagram below. 
Fic. 4.—Air-gauge giving pressure (after Amagat). 
This apparatus, filled with dry air, was allowed to come 
exactly to the temperature of the water inside the compression 
apparatus ; then, the open lower end of it being dipped into a large 
vessel of mercury, it was let down full of air into the compres- 
sion cylinder and pressure was applied. The effect was of 
course to compress the air, force up the mercury until it gradually 
filled the vessel and forced the air entirely into the smaller bulb. 
After a few trials we found roughly what amount of pressure 
was necessary in order just to commence the forcing of mercury 
into the small bulb. The mercury forced in was weighed ; then 
the capacity of the small bulb was determined by weighing its 
contents in mercury. ‘The difference of these weights is the 
weight of mercury, which would occupy the same volume as did 
the air when compressed. Finally, the original volume of the 
air was found by weighing the whole apparatus, first empty 
then filled with water ; and, most important in view of Amagat’s 
results, the barometer and thermometer were carefully observed 
at the instant when the apparatus had its lower end placed in 
the vessel of mercury. Mr. Kemp, who made these instruments 
for me, sugge-ted and carried out the great improvement of 
inserting a small triangular pyramid of glass into the choked 
part of the bore (as shown in the small sketch). The effect is to 
break the mercury (which must be very clean) into exceedingly 
small drops, In this way the actual compression of the air was 
determined with a limit of error, represented at the utmost by 
the ratio of the volume of one of the small drops of mercury 
formed at the obstruction to the whole capacity of the small 
bulb. By working simultaneously with three instruments of this 
kind, even this very small error could be in great part elimi- 
nated :—and, practically, the compressions were measured far 
* By Prof. Tait. Abridged by the Author from a forthcoming volume of 
the Reports of the Voyage of H.M.S. Chadlenger, by permission of the 
Lords Commissioners of H.M. Treasury. Continued from p. 93- 
2 ‘*Mémoire sur la Compressibilité des Gaz & des pressions élevées,’’ par 
M. E.-H. Amagat (Ann. de Chimie et de Physique, 1880). 
more accurately than was at all necessary for the purpose in 
hand. For greater accuracy a larger apparatus would be re- 
quired. This, however, was quite unnecessary. And the 
requisite limit of accuracy in the experiment rendered it un- 
necessary to correct for the alteration of volume of the smaller 
bulb consequent on the pressure to which it was subjected, 
In my later experiments a long carefully-gauged tube of 1°5 
mm. in bore was substituted for the small bulb. This tube was 
coated internally with an excessively thin film of metallic silver 
thrown down by sugar of milk. The process was arrested the 
moment the film became visible by reflection. This film is at 
once dissolved by the mercury up to the point which it reaches 
at the greatest pressure, and leaves a perfectly sharp and nearly 
opaque edge from which to measure, This device has proved 
so very successful that 1 have now substituted it for the indices 
in all the pressure gauges (shortly to be described) which are 
employed for very accurate measures. And I am at present 
engaged in measuring, by comparison of a glass gauge and an 
air-gauge both filled in this manner, the compression of various 
gases at pressures up to fourfold those applied by Amagat. 
XII. Znternal Pressure Gauges—The next step was to find 
some plan of construction for an instrument which, having its 
scale determined once for all by comparison with the air-gauge, 
should ever afterwards serve instead of it, thus affording a ready 
measure of pressure. Liquids are obviously better fitted for this 
purpose tian solids, if only on account of their absolute homo- 
geneity and their greater compressibility. But, unfortunately, 
two liquids must be employed, since a record must be kept :— 
the apparatus being surrounded on all sides by nine inches of 
iron :—and all my trials with two liquids were more or less 
unsatisfactory. The very fact that I was dealing with thermo- 
meters whose bulbs were protected from pressure, at once 
suggested an unprotected thermometer as something perfectly 
well suited to the purpose so long as the glass might be trusted 
to follow Hooke’s law. [I have since found that the invention 
of such an instrument, to be used as an é/a/érométre, is due to 
Parrot.! His investigation of the effects of pressure is wholly 
incorrect, as it takes no account of distortion ; but the device, 
and the recognition of the fact that its indications are propor- 
tional to the pressure, are wholly his.] 
These instruments, which, like the thermometers, are fitted 
with a needle-index with hairs attached, have only one defect, 
which is that they act like thermometers as well as pressure- 
gauges. That defect I managed to remove almost completely 
by the simple device of inclosing in the bulb a closed glass tube 
which a// éut fills it. The liquid then occupies only a small- 
space between the interior tube of glass and the exterior tube 
forming the bulb, and is as ready as ever to give indications of 
pressure, while it is not in sufficient volume to be more than 
slightly disturbed even by a serious change of temperature. 
Fic. 5.—Internal gauge, plugged. 
It is quite easy, by comparing two instruments of this kind 
in which the ratios of the internal to the external radius of the 
cylindrical bulb are different, to find by trial through what range 
its indications are strictly proportional to the pressure. Thus 
all the requisites of a perfect gauge, so far as the experiments 
required, were met by this simple apparatus. That I have 
obtained a sufficient accuracy in the graduation of these instru- 
ments is proved by the cluse agreement between my results for 
the volumes of air at different pressures as measured by means 
of them, with the volumes corresponding to these pressures in 
Amagat’s table. If Boyle’s law had been eyen approximately 
true for these high pressures, this mode of verification would 
have been fallacious. It would, however, be easy to make an 
independent verification, by sinking some of these instruments, 
each thoroughly imbedded in a mass of lard (as a protection 
1 “(Expériences de forte compression sur divers corps, par M. Parrot’ 
(Mémoires de l’ Académie Impériale des Sciences de St. Petersbourg, 6me. 
série, tome ii., 1833). The pages are headed s¢ Parrot et Lenz,” and it was 
by mere accident (seeking in the Royal Society's “Catalogue of Scientific 
Memoirs’ for a reference to Lenz's thermo-electric writings) that I lit on 
the paper. I was much surprised at some of the statements it ccntains, till 
I found at the very end a footnote by Lenz, in which he disclaims all respon- 
sibility for the writing of the paper, and for the conclusions drawn in it. 
