344 GRISCOM ON METEOROLOGICAL OBSERVATIONS, 



consider the most favourable, would be a post, placed at some distance 

 from any building which might reflect heat, and surrounded, in the 

 warm season, to the distance of ten or twelve feet by verdure. A 

 sloping roof should encompass the post to protect the instrument from 

 direct rays, but not so as to prevent a free and uninterrupted circula- 

 tion of air. Beside an instrument of the ordinary construction, thus 

 situated for determining the mean temperature of the air, as deduced 

 from three stated observations in every twenty-four hours, viz. at 8 

 o'clock, a. m., 1 p. m., and 6 p. m., we would recommend that the 

 society procure, if possible, a good Six's, or self-registering thermome- 

 ter, by which the greatest heat of the day, and the greatest cold of the 

 night, can be determined by bare inspection, several hours afterwards. 

 The highest intensity of solar heat, as indicated by the thermometer, 

 is obtained when the instrument is protected by a box of glass from 

 aerial agitations, and exposed to the direct influence of the sun beams. 

 The posterior side of the box may be of wood, and its internal surface 

 should be painted black, or lined with black cloth. The highest ele- 

 vations of an instrument thus adjusted, during our hottest weather, 

 would, if carefully registered, form an interesting addition to the obser- 

 vations usually noted. 



2d. Changes of atmospheric pressure* 



We have only to recommend that the barometer employed for this 

 purpose be a good one; that it be furnished with a nonius for reading 

 off the elevations to hundredths of an inch ; that the figure of the mer- 

 curial surface be noted, with the elevations, whether concave, convex, 

 or horizontal; and that, in addition to the regular observations, the 

 height of the quicksilver in this instrument be attended to as far as the 



