Mr. W. S. Jevous on the Forms of Clouds. 243 



a section-glass, because it contains a thin section of liquid which 

 is supposed to represent a section of the atmosphere. It con- 

 sists of two sheets of plate-glass (about 18 inches by 14), which 

 being let into two plain wooden frames, can be so screwed 

 together, face to face, as to form a water-tight vessel with an 

 internal space of the uniform width of about one-third of an inch. 



This being: filled with water, we have a thin layer or section of 

 liquid, of v/hich the minutest motions or changes can be conve- 

 niently detected and observed either by means of the small par- 

 ticles of sediment floating in it, or by the production of a preci- 

 pitate of chloride of silver, of which the component parts, silver 

 and chlorine, are contained in distinct portions of the liquid. 



One or more glass tubes may be introduced in any desirable 

 positions into the section-glass through grooves cut in the oppo- 

 site faces of the wooden frame, and numerous currents of liquid, 

 varying in velocity, volume, or quantity, direction, temperature, 

 density, &c., may thus be introduced, so that their complex mo- 

 tions and reactions may be observed in the most convenient 

 manner. It is very easy to render the whole water-tight, if a 

 thin continued band of common glazier's putty be laid round 

 between the level surfaces of the wooden frames before they are 

 screwed together, the tubes required for the experiment in pre- 

 paration being likewise imbedded in the same. One of the 

 glass plates has been faintly engraved, by means of hydrofluoric 

 acid, with two series of rectangular lines at the uniform distance 

 of one inch ; these afford in every part a fixed line of reference 

 to detect the slightest movements of the enclosed liquid, as well 

 as to measure the rate or extent of that movement if required. 



The accessory apparatus, such as tubes, funnels, stopcocks, 

 &c., will at once be understood from Plate II. fig. 1, in which 

 the instrument is arranged as it is chiefly employed. 



_ 6. The liquids used consist of the following dilute solutions, 

 viz. — 



(1) "Water containing 1 part in 5000 of common salt. 



(2) Water containing 1 part in 5000 of nitrate of silver. 



(3) An aqueous solution containing one-tenth of its weight of 

 ordinary white sugar. 



The first two have about the same specific gravity, but pro- 

 duce, when they mix, a white precipitate of chloride of silver, 

 which, though perfectly distinct, is so thin and impalpable as to 

 remain a long time suspended in the water, the motions of which 

 its weight docs not perceptibly influence. Known quantities of 

 the third solution are, however, added to the two former, with 

 the object of producing such differences of specific gravity as 

 form the required conditions of the experiment. 



7. A little reflection will show that the whole variety of phse- 

 R2 . 



