Mr Harvey on the Formation of Dew. 09 



edges' of combination between P and P + l, which, in a form 

 merely consisting of | and P+l, will produce Fig. 29, a va- 

 riety which has been found in Cornwall. 1 have lately ob- 

 served a composition of this kind, similar to Fig. 34, likewise 



from Cornwall, in Mr Allans collection, the form of the indivi- 



p 

 duals contained in the group consisting of P — no • — 2-P+ 1 * 



P+ao , and several other simple forms which it was impossi- 

 ble to determine with any degree of accuracy, on account of 

 the numerous and deep furrows upon the faces, parallel to 



p 



their intersections with j and P + l, and the want of lustre of 

 the surface, which prevented the application of the reflective 

 goniometer. A simple crystal of the form represented Fig. 

 35, occurs upon the specimen which contains the compound 



varieties. 



(To be continued.) 



Art. X. — Facts relating to the Formation of Dew. By 

 George Hahvey, Esq. F. R. S. Lond. & Edin. Commu- 

 nicated by the Author. 



The tower of St Andrew's Church, Plymouth, is situated 

 about 500 yards to the east of the meadow, in which I have 

 usually performed my experiments on the interesting subject 

 of dew, and the elevation of its summit above the level of the 

 field, about 110 feet. For the purpose of tracing the law 

 which regulates the deposition of dew at different altitudes 

 above the surface of the earth, I frequently found it necessary, 

 in conjunction with Mr Pridham, to perform analogous expe- 

 riments, at the same time, on the top of the tower, and on the 

 surface of the meadow ; and I select, from many interesting 

 results, that obtained on the night of the 21st of May, on ac- 

 count of the remarkable states of equality which were observ- 

 ed, both in the temperature of the air, and in the quantities 

 of moisture deposited on bodies of the same kind, when placed 

 on substances having different radiating powers. 



The night wassereneand tranquil, but the sky not remarkable 

 for its clearness. The first observation wasmade at 10 r. m., when 

 the air, at the summit of the tower, and at three feet above the 



