236 



quency is injurious, inasmuch as the cloudiness cuts off the influence 

 of sunshine. The fact that years of low temperature are always 

 years of poor crops is a fact that must be generally considered as a 

 local phenomenon because of the simultaneous conpensation as to 

 temperature that is continually going on in contiguous localities. 



HOFFMAN. 



Prof. Dr. H. Hoffmann published, first at Giessen and afterwards 

 in the Memoirs of the Senckenberg Association at Frankfort (Vol. 

 VIII, 1872), the details of a work which he began in Giessen in 1866 

 on the relation between the development of plants and the tempera- 

 ture recorded by a maximum thermometer in full sunshine. Some 

 account of that work and its subsequent continuation at Giessen is 

 given in successive papers published in the Journal of the Austrian 

 Meteorological Association (Zeitschrift O. G. M.) during the years 

 1868 to 1891. The detailed references to these will be found in the 

 list of papers af)pended to this present report. Hoffmann's first 

 conclusion, as stated in 1868, was that he had found a precise, intel- 

 ligible, and comparable expression for the quantity of heat that is 

 needed for the attainment of any definiie phase of vegetation. He 

 would take the sum of the daily maxima of a thermometer fully ex- 

 posed to the sunshine. His first work at Giessen was done with a 

 naked glass bulb, self-registering, mercurial, maximum thermometer, 

 graduated to Reaumur's scale, attached to a wooden frame and set 

 out in full sunshine 4.5 French feet above the soil or green sod in an 

 open portion of the botanic garden at Frankfort. The exposure was 

 indeed not perfectly free, but was such that the sun shone upon the 

 thermometer from sunrise to 2 p. m. in January and until 4.30 p. m. 

 in June. Hoffmann's summations begin with midwinter, or January 

 1, and he gives the sums of the positive daily maxima (i. e., above 

 0° Reaum. ) up to the dates of leafing and flowering for 10 plants. 



Apparently preliminary values are given in the Journal of the 

 Austrian Meteorological Society for 1868 and 1869, but final values 

 in the memoir published at Frankfort, 1872. 



In the Meteorologische Zeitschrift for 1875 Hoffmann says that 

 after four years' work at Giessen (1866-1869) his thermometer was 

 broken. A new one was constructed by Dr. J. Ziegler, of Frankfort, 

 in accordance with their mutual understanding; this had a mercurial 

 bulb, but was very many times larger than the former, and therefore 

 very much more sluggish. Observations with such instruments, 

 graduated to accord with the Reaumur scale, were begun in 1875 by 

 Hoffmann at the botanic gardens at Giessen, and by Ziegler at the 

 gardens at Frankfort. In order to compare these two series together 

 and to unite them with the earlier Giessen series the ratios of the 

 sums as given by the earlier and the later thermometers for the same 



