TRANSACTIONS OF THE SECTIONS. 



53 



micrometer-screw M. The action of the lever is so adjusted that, when the tele- 

 scope is moved through any angle, it causes the last prism to tiirn through double 

 that angle. The rays which issue from the centre of the last prism are thus made 



to fall perpendicidarly upon the centre of the object-glass of the telescope T ; and 

 thus the ray of light travels parallel to the bases of the several prisms, and ulti- 

 mately along the optical axis of the telescope itself, and thereby the whole field of 

 the object-glass is tilled with light. 



Thus the apparatus is so arranged that, on turning the micrometer-screw so as 

 to make a line in the spectnmi coincide with the cross wires in the eyepiece of the 

 telescope, the lever L, attached to the telescope and prisms, sets the whole of the 

 prisms in motion, and adjusts them to the minimum angle of deviation for that 

 portion of the spectrum. 



On the Examination of Sea Water on hoard IT.M.S. 'Porcupine,^ in July 

 1870, for dissolving Gases and varying proportions of Cldorine. By W. 

 Lant Cakpenter. 



Contributions to Mineralogical Chemistry. By A. H. Chtjech, M.A,, F.C.S. 



Eooperinients on the Preservation of Stone. By A. H. CnuECH, M.A., F.C.S. 



On the Pm-ijlcation of Public Thoroughfares by the application of Deliquescent 

 Chlorides. By "W". J. Coopee. 



The author had first called attention to the subject in 1868 ; at that time a very 

 successful experiment had been tried in Baker Street, Portman .Square. In Liver- 

 pool, in 1869, Bold Street, Church Street, and Lord Street were watered with salts 

 during the month of July. The report of the result was A-ery favourable, and the 

 experiments have been continued this year. In many towns experiments liavo 

 been tried during the past season, with various results, according to tl e composi- 



