6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



ID. For results not requiring the highest accuracy the above de- 

 scribed method of reading is satisfactory. As the result of long ex- 

 perience, we now use for greater accuracy several refinements of this 

 method, as follows : 



(a) A special reading glass is used. It consists of a small eyepiece 

 of about 4 cm. focal length, mounted so that it can easily be held 

 against and moved along the thermometer stem. In the focus exactly 

 in the center of the field is a sharp needle point. By taking readings 

 when the needle point is opposite the top of the mercury column, 

 parallax errors are eliminated. 



(b) Any simple device to beat regular intervals (such as i, 5, or lo 

 seconds) permits the observer to concentrate on reading the ther- 

 mometer instead of trying to read both watch and thermometer at the 

 same time. Such a device also eliminates possible error due to eccen- 

 tricity of the second hand of the watch. 



(c) The instrument is set out in the sun at least 15 minutes be- 

 fore starting to read, and the shutter opened to the sun for about i 

 minute during this period. In making a series of observations, the 

 second set of readings is started 20 seconds after completing the 

 first set of six readings. Thus a 4-minute shaded period occurs be- 

 tween each 2 minutes of exposure. Each set of six readings is quite 

 independent. 



13. Take the algebraic means 



(i)-(2)-K5)-(6) 

 2 

 and to them add [(4) — (3)]. 



14. Call this result Ri. Find roughly the mean temperature Ti dur- 

 ing the interval of exposure (3) to (4). 



1 5. Add to Ri the percentage correction for graduation furnished with 

 the instrument, then, after correcting, add to Ri, K[(Ti — 30°)Ri]. 

 K is a constant furnished with the instrument. If the prevailing tem- 

 perature of the air differs much from 20°, add 0.0014 R for each 10° 

 the air temperature falls below 20°. The result (which we will call 

 R^i) is the final rate of rise per 100 seconds during the exposure 

 (3) to (4) as reduced to the standard bulb temperature of 30°, and 

 standard stem temperature of 20°. 



