METEOROLOGY. 



555 



the effect of the motion of the air, which latter has therefore generally 

 been quite left out of consideration. {Z. 0. G. M., xvn, p. 56.) 



Hoffman, in continuation of his long series of studies into vegetable 

 physiology, states that in the determination of the thermal constants 

 of vegetation the summation of the daily maximum positive readings 

 of a thermometer in sunshine gives far better results than the use of 

 the ordinary air temperature, since the plant itself is mostly exposed to 

 the full sunshine. The error of the air temperature method is often 10 

 per cent, of the total temperature sum, while that of the sun thermome- 

 ter but 1 per cent. The errors of the former method are least in low 

 and shady places, but become very large when applied to high and 

 mountain regions, where the shaded thermometer always reads much 

 lower than the sunshine thermometer. By using sunshine or insolation 

 thermometers of perfectly uniform size and spherical bulbs, Hoffman 

 hopes to make the observation at neighboring stations more perfectly 

 comparable, and refers with satisfaction to the progress made in this 

 respect by Dr. J. Ziegler, of Frankfort. [It would seem that the ther- 

 mometers used are ordinary bright bulb with a brass scale and not in 

 vacuo, and which always agree within 2° centigrade. Whatever objec- 

 tion there may have been to the bright and black bulb in vacuo, or the 

 Arago-Davy actinometer, as a means of obtaining the relative intensity 

 of sunshine, have, I think, been mostly removed by recent researches 

 of Prof. Ferrel, now in press, audit would seem that this instrument 

 should be substituted for the thermometer in air.] 



The comparative observations made by Hoffman at low and at ele- 

 vated points show clearly the effect of superincumbent air, mist, haze, 

 smoke, &c. ; thus the differences between the reading of the same ther- 

 mometer exposed ten or twenty minutes in the sunshine and then in 

 the shade in calm and under very clear skies, gave for Giesen, altitude 

 402 feet, 4°.0 centigrade, while observations on mountains 5,000 to 8,000 

 feet high, give 1G°.4 centigrade. Selecting only observations made on 

 the same day as given by Hoffman, I have compiled the following 

 nearly comparable results : 



