Prof. Callan on a Contact Breaker. 255 



particles takes place, the ascending body of air will not be so 

 niucli checked, and having lost its tendency to redescend, as in 

 the cumulus, will spread out in stratiform projections, and pro- 

 duce the cumulostratose form. 



4. If subsidence and aggregation of the cloud-particles into 

 rain continue and increase, the air throughout the cloud will be 

 gradually relieved from the weight of the water, yet retaining 

 the latent heat evolved in its condensation, will decrease in spe- 

 cific gravity. It will, therefore, commencing with the upmost 

 parts, tend to ascend through the superincumbent strata, pro- 

 ducing (1) the cirrose crest which is characteristic of the 

 nimbus. 



5. The electricity of a thunder-cloud is continuously derived 

 from the weak charge previously existing in the ascending cur- 

 rent which supplies the moisture. The precipitated watery par- 

 ticles collect this electricity, which, as they gradually aggregate 

 into drops of rain, becomes intensified to an indefinite extent. 



EXPLANATION OF PLATE II. 



Fig. 1. Section-glass. Figs. 3 and 4. Cumulostratus. 



Fig. 2. Cumulus. Fig. 5. Tiiunder-cloud. 



The letters in the figures denote : 



L. Liquid of less or least specific gravity compared with the other por- 

 tions of liquid with which it is in contact. 



G. Liquid of greater or greatest specific gravity. 



I. Liquid intermediate in specific gravity to two other portions of liquid. 



Incr. Liquid of which the specific gravity is increasing. 



Deer. Liquid of which the specific gravity is decreasing. 



N.B. Figs. 2, 3, 4, and 5 are engraved from photographs of the actual 

 experiments. 



Royal Mint, Sydnev, New Soutli "Wales, 

 January "9, 1858. 



XXXII. A Description of a Contact Breaker superior to any hitherto 

 made, and of certain Effects of a Condenser on the Action of 

 various kinds of Contact Breakers. By the Rev. N. J. Callan, 

 D.D., Professor of Natural Philosophy in Maynooth College'^. 



IN 1837 I constructed a contact breaker in which the contact 

 was made and broken by a copper wire which was caused 

 to vibrate very rapidly, so that its opposite ends which were amal- 

 gamated dip])cd alternately into mercury. A description of this 

 instrument was published in Sturgeon's 'Annals of Electricity,^ 

 &c., in Aj)ril 1837. It was the first contact breaker ever made; 

 it was not self-acting. When the mercury is covered with oil, it 

 is superior to all other non-self-acting contact breakers. 



♦ Cominunicated by the Author. 



