476 Proceedings of the Royal Irhh Academy. 
and that as these sugars have opposite rotatory powers, the amount of 
rotation produced by their presence is only the difference between the 
separate effects. 
It is easy to verify the former of these suppositions by submitting 
the beet syrup uninverted to the copper test. If it be found that any 
appreciable amount of the copper is reduced, it will be necessary to 
have recourse to the more general method. 
Referring more especially to the sugar beets grown in Ireland in 
1872, the author said that an examination of these beets was impor- 
tant, because the atmospheric conditions of the past year were, at least 
according to the ordinarily received opinions, peculiarly unfavourable 
to the development of sugar. The amount, therefore, obtained from 
the beets of 18*72 might be regarded as a minimum ; unless indeed it 
be a mistake to suppose that hot, dry weather is essential to the sugar- 
producing qualities of the beet. Under either supposition the rest of 
the experiment has evidently an important bearing upon the question, 
whether the sugar beet can be profitably grown in Ireland. 
The accompanying table contains the result of four specimens of 
sugar beet grown upon the Albert farm, Glasnevin, in the year 1872. 
No. of root. Manure used. Water per cent. Sugar per cent. 
1. Common Salt, . . . 79-99 . . 12-72 
2. Sulphate of Potash, . 80-27 . . 13.18 
3. No manure, .... 80*60 . . 12*42 
4. Sulphate of Ammonia, 80*52 . . 11*85 
Mean, 80-34 . . 12-54 
• If these results be compared with the results obtained by the 
author for the sugar beets of 1870, it will be seen that there is no dimi- 
nution in the quantity of sugar. It seems probable therefore that 
hot dry weather is not essential to the sugar-producing qnalities of the 
beet. 
XXXIX. — Descetption or a Compaeaele Hygeometee, which 
Registees the Maximum and Minimum op Siccity and Humidity 
of the Atmospheee in the absence of an Obseevee, with Obsee- 
vATioNS ON ITS EMPLOYMENT. By M. DoNOVAN, Esq. (With Plate 
XXy., Science.) 
[Read April 14, 1873.] 
To discover the presence and measure the quantity of invisible watery 
vapour contained in any volume of the atmosphere has long been a 
desideratum, for the attainment of which hygrometers have been 
invented. They are variously constructed, and generally of a material 
