468 Preservation of Green-house Plants. — Roman Cement. 



made by me at Unst, in latitude 60° 45' 28" north, give 

 •0053605 for the diminution of gravity from the pole to the 

 equator, and 3o|.t7x for the resulting compression. 



" If Mr. Dunlop's experiments at Paramatta be compared 

 with those made at London, we obtain 39'07751 for the length 

 of the seconds' pendulum at Paramatta, '0052238 for the di- 

 minution of gravity from the pole to the equator, and ^gy-^^ 

 for the compression. Or, comparin^o- Mr. Dunlop's experi- 

 ments with those made at Unst, we have '0053292 for the 

 dimiinition of gravity from the pole to the equator, and ^^jjx-o^ 

 for the resulting compression. 



" The compressions here deduced must not as yet be deemed 

 conclusive ; for it is well known that a very small alteration in 

 the number of vibrations made by the pendulum would occa- 

 sion a considerable difference in the fraction indicating the 

 compression. The indefatigable zeal of Sir Thomas Brisbane 

 will, however, no doubt soon furnish additional data." 



PRESERVATION OF GREENHOUSE-PLANTS. 



It has been ascertained, by Mrs. Tredgold, that plants may 

 be completely protected from the depredations of insects by 

 washing them with a solution of bitter aloes, and the use of 

 this wash does not appear to affect the health of the plants in 

 the slightest degree. And wherever the solution has been 

 used, insects have not been observed to attack the plants again. 

 As there is much difficulty in preserving a small collection 

 by the usual methods, this notice of a simple remedy may be 

 very useful. 



ROMAN CEMENT. 



According to an analysis lately made by M. Berthier, the 

 component parts of Parker's cement are : 



Carbonate of lime -657 



magnesia .... -005 



iron '070 



> ■ manganese. ... -019 



Clay silica -ISO 



alumuia • -066 



Water 013 



l-OOO 



M. Berthier is of opinion, that with one part of common 

 clay and two parts and a half of chalk, a very good hydraulic 

 lime may be made, which will set as speedily as Parker's ce- 

 ment. He concludes from many experiments, that a limestone 

 containing six per cent, of clay affords a mortar perceptibly 

 hydraulic. Lime containing from 1 5 to 20 per cent, is very 

 hydraulic; and when fi-om 25 to 30 it sets almost instantly, 

 and may therefore be held to be, to all intents and purposes, 

 real Roman cement. sir 



