barus] • PORCELAIN AIR THERMOMETRY. 167 
CONSTANT-VOLUME THERMOMETERS. 
In most of the present experiments the object has not been to test the 
rigorous accuracy of the air thermometer so much as to devise forms 
of apparatus in which such tests can be satisfactorily made. I have 
had in mind, moreover, that the thermometer is to be used as a means 
of calibration the thernio-element. Having found therefore that at 
temperatures not exceeding 1,300° the porcelain of Bayeux is quite 
rigid as regards excesses of pressure (internal or external) not exceed- 
ing one atmosphere, I made the early air-thermometer measurements by 
the constant- volume method. • Jolly's 1 well-known and convenient ma- 
nometer was largely used, with such modifications as the special work 
required. To connect bulb and manometer of the air-thermometer adjust- 
ment I used capillary metallic tubes. Such tubes had been used by 
Keguault 2 and others before. They enable the observer to place the 
nianometric apparatus at some distance from the furnace and the bulb, 
a condition of accurate measurement which, at high temperatures, is 
almost essential. I will briefly describe the apparatus more in detail. 
The general disposition of apparatus is given in Fig. 29 (frontispiece), 
and there will be little difficulty in recognizing the parts to which the 
descriptions refer. Further comment is made below (p. 188). 
Manometer. — A very substantial form of manometer stand, made for 
the work by Mr. William Grunow, of West Point, is shown in the frontis- 
piece, under B. The stand is essentially that of Jolly, as modified by 
Professor Pfaundler in Insbruck. 3 It consists of two vertical parallel 
.cylindrical slides made of brass, 160 cm in length, 2.5 c,n in diameter, and 
about 13 cm apart, measuring from center to center. These slides (tubu- 
lar) are fastened below to a suitably large tripod support, and are 
braced above by a slender St, Anthony's cross, the lower end of which 
abuts against the tripod. Metallic clamps or sleeves, provided with 
suitable devices for carrying the manometer tubes, are thus free to slide 
along the whole vertical range of 100 cm . A millimeter scale, about two 
meters long, is placed between the brass upright for approximate meas- 
urement. Finer measurements are made with a cathetometer. 4 Obvi- 
ously this stand is equally convenient as a support for both the constant 
volume and the constant pressure apparatus, either of Avhich methods of 
air thermometry may be used with equal facility. The modification of 
the Jolly-PfauDdler- stand, by which the brace is made to abut against 
the top of the slides instead of against the middle, as in the old form, is 
due to Dr. Hallock. I have modified the Jolly-Pfaundler stand by using 
four parallel slides of about the length and distance apart of those just 
described (Fig. 43). The two front slides are used for the manometer 
1 Jolly: Poggendorff Annalen, Jubelband, 1874, p. 82. 
2 Regnault: Relation des experiences, Paris, 1847, p. 264. 
3 Cf. Miiller Pouillet, Physik, vol. 2, 1879, p. 119. 
4 Mr. Grunow's form. 
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