68 Royal Institution : — 



notwithstanding the great depression of evaporating surfaces. 

 Daring* the night hours in March 1850, the mean depression 

 of the temperature of the wet-bulb thermometer varied from 6°*0 

 in the dampest period, to 9°*9 in the driest. During the day- 

 hours the mean depression varied from 12 0, 4. in the dampest, to 

 15°0 in the driest. If the cooling of evaporation had any im- 

 portant effect at night in increasing the fall of temperature, it 

 must have acted during the day in diminishing the rise of tem- 

 perature. But the rise was in no sense diminished, but the 

 reverse ; therefore it may fairly be inferred that the action of 

 evaporation in causing night cold must be unimportant, — a con- 

 clusion which otherwise would seem probable. 



I am aware that Dr. Tyndall's researches have indicated that 

 the watery vapour in the air is less effective in impeding the 

 passage of solar heat than that of terrestrial heat. But the facts 

 now noticed seem to show that even on solar heat the effects are 

 important. 



It will be understood that these remarks are only intended to 

 indicate conclusions come to on the consideration of a limited 

 field of observations. Should I have an opportunity of finding 

 other facts of a like nature likely to throw further light on this 

 important matter, I shall endeavour to publish them also. 



X. Proceedings of Learned Societies. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



June 15, " T7XPERIMENTS on the Vibrations of Strings." By 

 1866. E* John Tyndall, Esq., LL.D., F.R.S., &c. 



1. I lay hold of one end of this india-rubber rope, the other end 

 of which is fixed to the ceiling, and by a jerk raise a protuberance 

 upon it. The protuberance runs along the rope to its fixed end, is 

 there reflected, and reversing itself, returns to my hand. In this 

 case, where the points of the rope rise in succession to form the pro- 

 tuberance, we have an example of a progressive wave or undulation. 



2. After the first wave I now send a second, so that it shall meet 

 the reflected wave on its return. The foremost ends of both waves 

 now meet in the centre of the rope ; they there neutralize each other, 

 and the two halves continue to swing with an apparently motionless 

 point called a node between them. 



3. I now stop the rope, send a wave forward, and then another 

 wave so quickly after it that this second wave shall meet the first at 

 one-third of the length of the rope from its fixed end. At that 

 point a node is produced. But I have already sent a third wave 

 after the second. The second wave being reflected at the node, 

 meets this third one and a second node is formed. The whole rope 

 is now divided into three vibrating parts, separated from each other 

 by two nodes. 



4. By properly timing the impulses imparted to the rope I can 

 divide it into four, five, six, ten, and even twenty vibrating parts 



