Observations on Visible Vibration. 15 



receives the impress of the figure, and from the great mobi- 

 lity of the liquid, minute changes are by its means detected. 

 Analogous figures have been obtained from the secondary 

 tones, the points of the star always increasing in number as 

 before stated. Similar results have to a certain extent been 

 obtained when vessels of porcelain, crokery-ware, and metal 

 have been employed ; this part of the subject will be re- 

 sumed. 



103. Another excellent method of ascertaining the num- 

 ber of nodes for any given note on a foot glass of any 

 shape, is by employing a limpid oil such as linseed oil.* 

 This is incomparably superior to water when employed 

 without lycopodium, as it does not vibrate and no splashing 

 whatever results. The glass may be about half filled with 

 the oil, care being taken in pouring the fluid to preserve 

 the surface of the glass above the oil perfectly clean and 

 dry. The note must be produced by a strong and rather 

 sudden stroke of the bow, the pressure of the latter being 

 increased in drawing it down, but at the same time the glass 

 must not be shaken. The curved line under these circum- 

 stances will be distinctly and perfectly marked round the 

 glass by the elevation of the waves. The fundamental note 

 will yield a line of four undulations, while for the secondary 

 tones the lines will consist respectively of six, eight, ten, 

 and twelve undulations. 



104. A figure can also be impressed upon the surface 

 of mercury contained in a foot glass by means similar to 

 those employed for water. A very slight coating of lyco- 

 podium being given to the mercurial surface, it was strongly 



* The peculiar whining tone produced by lowering the tension of a string 

 during vibration is well known. This effect can be imitated by strongly vibrating 

 a glass and suddenly pouring in water ; the note is lowered in proportion as water 

 is added, and the whining eflfect is produced by the note passing rapidly through 

 a series of tones to that tone produced by the whole bulk of water added, and the 

 glass still continues to vibrate for some time after the addition of tlie water. It 

 is a curious fact that if oil instead of water be added to the vibrating glass the 

 effect of that fluid is to stop vibration instantaneously, but if oil heated to about 

 250° be added to the vibrating glass, the note continues precisely as if water had 

 been employed. The cohesive nature of oil will explain tliis, the eflfect of heat 

 being to overcome the cohesion of its particles so as to enable them to glide more 

 easily over each other, and consequently not to become attached to the surface of 

 the glass. The effect of heat upon oils has been already shewn (23 et seq.) and 

 this experiment is analogous. 



