176 WRITINGS OF JOSEPH HENRY. [1855- 



the degrees may readily be divided into small fractions; but 

 in this case the thermometer will fall behind in its indica- 

 tions, since if the temperature be increasing, some time 

 must elapse before the instrument can arrive at this new 

 condition ; and in case it be falling, a similar tardiness will 

 be exhibited. If on the other hand the bulb be very small, 

 the degrees will be of less length ; but since there is little of 

 the fluid to be heated or cooled, it will more readily take 

 the temperature of the circumambient air. For determining 

 however the mean temperature of a place, the thermometer 

 should not be too small, since in that case it will be more 

 easily affected by the heat of the body during observation, 

 and at the same time it may be affected by an accidental or 

 fitful stream of air, and thus give too high or too low an in- 

 dication. One of the ordinary size in which the bulb is 

 about half an inch in diameter, is preferable. 



For a similar reason the thermometer ought not to be 

 suspended in immediate contact with a large solid conduct- 

 ing body, for example a stone or brick house, since this will 

 retain the effects of a term of heat perhaps for several hours 

 after the temperature of the air has changed. It should be 

 suspended from an imperfectly conducting material, such as 

 wood, and so situated that the air may circulate around it 

 on every side. It should also be screened from the direct 

 radiation of the sun, and from the reflection of surrounding 

 bodies; for if this be not done it will indicate the average 

 of all the impressions received, and not simply the tempera- 

 ture of the air. The thermometer therefore ought to be 

 placed in the shade on the north side of the house, but a few 

 feet above the level of the ground, in an unobstructed place; 

 and indeed it has been recommended to suspend it between 

 two large parallel horizontal discs of wood, which will protect 

 it from the earth below, the sky above, and every influence 

 except that of the stratum of air in which it is situated. 

 Instead of this however, we may enclose it in lattice-work, 

 easily permeated by currents of air, and painted white on the 

 outside to reflect back the more intense rays of heat which 

 may accidentally reach it. 



