58 BULLETIN 1136, U. 8. DEPABTMEST OF A&RlcrLTTTBE. 
comparisons can well be ruacle during the rim, if care be taken to 
read rlie instruments only after the temperature has been practically 
constant for 10 minutes, to allow the recorder bulb to overcome its 
natural lag. A full correction curve can then be plotted for the 
entire range, and the pen arm of the recorder adjusted to reduce this 
correction to the minimum. For making this adjustment there is 
usually provided a small screw at or near the pen-arm pivot, the 
turning of which moves the pen over the scale. 
TTet-bulb recorders should be calibrated similarly, preferably with- 
out the wick, and double-pen instruments should have both bulbs 
calibrated, dry. at the same time. An occasional check with a wet 
and dry bulb thermometer will show whether the wet bulb is really 
recording the wet -bulb temperature. It must be kept in mind that 
a reasonable amount of circulation past the bulb is necessary to 
secure enough evaporation to bring the bulb temperature clear down 
to the actual wet-bulb temperature. 
Eeeorders should be calibrated in place at least once every two 
months, and oftener if they show a tendency to fluctuate abnormally. 
They should be handled carefully, in accordance with the manufac- 
turer's instructions, special care being taken in changing charts not 
to bend the pen arm. and when filling the pen not to spill ink down 
the pen arms. Instruments should be returned to the manufacturer 
when other than the clock mechanism needs repair. Competent 
jewelers can keep the clocks in order. 
Although recorders can be obtained in weatherproof cases which 
need no special protection xrom the elements, it will be found advan- 
tageous to mount them in the >perating room in some place which 
i£ readily accessible and as free from temperature changes as 
possible. 
THERMOSTATS. 
Thermostats do not as a rule require any calibration except to 
determine whether in the case of the wet bulb there is enough circu- 
lation past it to insure proper depression. This can be done by 
comparison with a wet-bulb thermometer placed right at the regu- 
lator bulb. It should be read first without fanning and again after 
vigorous fanning. The drop in temperature will indicate the extent 
to which the regulator bulb represents the actual wet-bulb temper- 
ature. 
It is necessary, however, to give the thermostat regulators occa- 
sional attention. In self-contained instruments which have a stuff- 
ing box on the valve stem a small quantity of oil and graphite ap- 
plied occasionally on the stem at the box will help to reduce fric- 
tion and make the instrument more accurate. The stuffing box 
should be tightened only enough to prevent leakage. 'In the air- 
operated type the small valves in the regulator head must be kept 
free from the oil whi : :i is apt to be carried by the air. An occa- 
sional washing of the head, by disconnecting the air lines and pour- 
ing gasoline through it. will keep the parts clean and prevent 
sticking. 
BPvYTXG OVEX. 
The drving oven needs- no particular attention, except to make 
sure that it is maintaining a temperature of 212 c F. and not vary- 
.-. : 
or 
