26 ' REPORT — 1851. 



it was not changeable like aurora borealis, for it invariably receded to a certain 

 dimness, and then brightened up to a certain extent. 



Epoch 1850. March 7d. 8h. Om. Not so brilliant. 



Epoch 1850. March 8d. 7h. 15ra. Pale and confused. 



Epoch 1850. March 9d. 7h. 55m. Brilliant for a few minutes. 



Epoch 1850. March lOd. 7h. 50m. Brighter than last night. 



Epoch 1850. March lid. 7h. 59m. Fainter. 



Epoch 1850. March ]2d. 8h. Om. Fainter. 



Epoch 1850. March 13d. 8h. Om. Yet fainter. 



Epoch 1851. January 22d. 6h. 30m. Apparent, but confused. . 



Epoch 185) . January 23d. 7h. Om. Tolerably briUiant by fits. 



Epoch 1851. January 27d. 7h. Om. Brilliant by fits. 



Epoch 1851. March 20d. 8h. Om. Edges well-defined. Extent of base on 

 horizon 23° 30'. The S. edge cut horizon 1° 30' S. of W. ; the N. edge 22° N. of 

 W. ; axis 15' N. of Pleiades, and horizon 10° 15' N. of W. 



Epoch 1851. March 22d. 8h. Om. Pale and confused. 



The evenings in February and March were much clouded in 1851. 



On Air-bubbles formed in Water, By John Tyndall, PJi.D. 



The object of the paper is to account for the bubbles formed when water is poured 

 into water. Venturi imagined that they were due to the air which adheres to the 

 descending fluid. Magnus, on the contrary, conjectured that they were owing to 

 the formation of a cavity at the point where the descending water meets the fluid 

 underneath, the cavity being closed up, and the bubble formed when a motion is 

 imparted to the surface. This latter conjecture the author proves to be correct, and 

 shows further the relation which subsists between the production of the bubbles and 

 the structure of the descending mass. 



A vein issuing at a moderate velocity from a circular orifice in the bottom of a tin 

 vessel possesses two portions strikingly distinct. After the fluid leaves the orifice it 

 is steady and limpid for some distance, and contracts gradually as it descends. The 

 contraction depends upon the accelerated motion of the particles, and its amount 

 depends upon the cohesive power of the fluid experimented with. At a certain point 

 the steadiness of the vein ceases, and it exhibits a quivering motion like the vibration 

 of a harp -string. If the surface of the water underneath intersect the vein above 

 this point no bubbles are produced ; below this point they invariably appear. The 

 steady portion of the vein exhibits the exact deportment of a solid rod ; there is even 

 capillar}' attraction at the point where it enters the fluid below ; and if the oblique 

 light of a candle be suffered to fall upon the eminence here formed, a moth-like 

 figure with yellow wings is thrown upon the bottom of the vessel which contains 

 the fluid. Similar figures are caused by the bubbles which float accidentally upon 

 the surface ; and when the latter is set in motion the figures stir their wings as if 

 they were alive. 



Although to the naked eye the quivering portion of the vein appears continuous, 

 it is not so in reality, as Savart has demonstrated. It is resolved into single drops, 

 and it is the quickness with which they succeed each other which gives the impression 

 of continuity. Placing a platinum wire heated to whiteness between the poles of a 

 small galvanic battery behind the vein, and using an opake fluid; the author found 

 that when the steady portion came between the eye and wire the latter was cut by 

 a dark stripe ; but when the quivering portion came into this position the wire glowed 

 uninterruptedly from end to end. It was seen through the intervals between the 

 drops, and the time of transit of a drop was not sufficient to extinguish the im- 

 pression. 



The descent of each drop causes the water to recede on all sides, thus forming a 

 canal into which the air enters, and where it is entrapped by the descent of the suc- 

 ceeding drop, or b)' the closing up of the mouth of the canal. Exactly the same 

 takes place when we let grains of shot fall successively into the water, or draw a 

 string of beads quickly through it. It is the lateral component of the ilrop's force 

 which originates the bubble, and even when the jet is continuous, if it vibrate, bub- 



