330 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
wanner tliau tliat in tlio inner, 
of the water in tlie inner an 
inner jar by contact witli the warmer air at its surface is very satisfactorily compensated 
by its loss through tlie air space between T> and C and the badly conducting walls of 
J>. For temperatures liigher than that of the air the Avater in the outer jar must be 
ISio po.sitive rules for differences iu the temperatures 
outer jars can be established, yet it may be said, iu 
general terms, tliat the greater the difference 
between the temperature of the air and that 
desired for comparison the greater should be 
the difference between the temperatures of 
water in the outer and inner jars. 
To avoid parallax error in reading, the jars 
were leveled and readings taken by aid of a hand 
lens, with the eye and top of the mercury column 
at the level of the top of the outer jar, across the 
two sides of which the reading is sighted, the 
thermometer being held in contact with one of 
the Avails of the jar and parallel Avith the central 
spindle of the frame to insure its ])erpendicu- 
larity. Comparisons of readings taken in this 
siui])le Avay Avith readings taken by the cathe- 
tometer, the thermometer being secured in a 
pci’iiendicnlar jmsition, show no i>erceptible 
error. When issued, each thermometer Avas 
accompanied by a. printed blank corresponding 
to a stub slip in the rating book and tilled out 
for each point at Avhich a. compari.son Avas made. 
The following admirable article (pp. 330-33G) 
upon the methods of thermometer coricction 
is kindly contributed by C. F. Maivin, professor 
of meteorology United States Weather Bureau, 
and atfords valuable information regarding the 
physical principles involved as Avell as their 
Cut 49.-Comp.Triiio.,iar for deep-sea l»r«^ctical application in the comparison and cor- 
theriiionieters. rectioii of thermometers. 
NOTES UPON THERMOMETERS AND HOW TO DETERMINE THEIR ERRORS. 
A clear understanding of the methods employed in determining the errors of 
thermometers can not bo obtained AAuthout a correct conception of the physical prin- 
ciples involved in the measurement of temperature and the action of thermometers 
in general. The following remarks will therefore preface a description of the methods 
employed at the United States Weather Bureau for comparison of thermometers: 
The soA^eral scales of units employed in the measurement of temperatures are 
all based upon tAvo definite temperatures at Avhich certain simple and easily reiuoduced 
physical phenomena invariably occur. The melting of ice formed from jiure water 
furnishes one of these temperatures, called, generally, the freezing ])oint. The second 
definite point in the scale of temperafures, namely, the boiling point, is established 
iu the teniiierature of the steam from pure boiling Avater. 
