THERMOMETER. 6 



weather. To prevent radiation from the room, the windows 

 during the intervals of observations may be closed with an in- 

 side wooden shutter. The outside of the lattice-work should be 

 painted white, to reflect off the light and heat which may fall 

 upon it.] 



The thermometer must be placed exactly perpendicular, the 

 middle of the scale being at the height of the eye against the 

 two small wooden bars, so that the top of the scale 

 being fixed by a screw to the upper bar, the bulb Fig- 2. 

 may pass at least two or three inches beyond the 

 lower bar. The instrument is attached to the last 

 by a little metallic clasp. (Fig. 2.) It will thus r,C 

 be placed ten or twelve inches from the panes, from \j n 

 the screen, and the other parts of the window. 



[In a later arrangement, a single transverse bar 

 is used. This being placed at the necessary height, 

 the thermometers are attached to it by small metal ("I 



brackets, which support them at a distance from the I , 



bar of about two inches. The metal brackets are 

 permanently screwed to the bar, and the thermome- 

 ters are fastened to them by small finger-screws, by 

 which they can be detached at pleasure. The order of placing 

 them is shown in the cut.] 



Reading. To read the thermometer, the eye must be placed 

 exactly at the same height as the column of mercury. Unless 

 this precaution is taken, there is a liability to errors, the greater 

 in proportion to the thickness of the glass of the stem and the 

 shortness of the degrees. The reading should be made at all 

 times, and especially in the winter, through the panes, and with- 

 out opening the window ; otherwise the temperature of the cham- 

 ber will inevitably influence the thermometer in the open air. 

 The degrees must be read, and the fractions carefully estimated 

 in tenths of degrees. After having rapidly taken the observa- 

 tion, another should be made to verify it. If there are several 

 other instruments to observe, and the thermometer is to be read 

 first, the first reading may be made some minutes before the hour; 

 the second, after the reading of the psychrometer ; and if there 

 is a difference, the mean number is to be entered in the journal. 

 When, notwithstanding the shelter, the bulb of the thermometer 

 is moistened by rain or fog, or covered with ice or snow, it is 



