Proceedings of the Polytechnic Association. 1053 



I^EW Dredging Appaeatus. 



Mr. J. Johnson tlien exhibited a model of a new apparatus devised 

 by himself for dredging. This dredger consists of an air-tight barge, 

 furnished with a large hole in the bottom, for the purpose of dis- 

 charging the load ; this hole being closed at other times by means of 

 an air-tight cover. The barge is filled by atmospheric pressure 

 applied as follows : From the deck of the barge proceeds a large 

 tube, which descends to the surface of the mud or other matter to be 

 removed. As soon as the barge is exhausted of air, the mud, &c., 

 will be forced up this tube, and will fall to the bottom of the barge. 

 The water is pumped out by the same means that the barge is 

 exhausted of air in the first place, and when a load has been secured, 

 the barge is towed off and emptied. 



April 8, 1869. 



Professor S. D. Tillman in the chair ; Mr. C. E. Emeby, Secretary. 

 Hygeometee. 



The discussion of new improvements being announced as in order, 

 Mr. Johnson introduced a very covenient form of hygrometer, with 

 a table of relative temperatures and degrees of humidity attached. 

 The instrument is generally known as the hygrodeik, and fully 

 described in the volume of the transactions of the Institute for 1866-7, 

 page 621. A good deal was said about the instrument being self- 

 registering. This, however, is an error, there being no registering 

 apparatus connected with it. The hygrodeik is little else than a 

 mason's hygrometer, the tables which usually accompany the latter 

 instrument being reduced to the form of a diagram, on which a 

 pointer indicates the results. From the pamphlet which was presented 

 by Mr. Johnson, we clip the following : Edson's hygodeik indicates 

 the state of the atmosphere in relation— 



1st. To its actual temperature as indicated by the ordinary thermo- 

 meter (or the dry bulb thermometer of this instrument). 



2d. Sensible temperature, or the temperature due to evaporation 

 (indicated by the wet bulb thermometer of this instrument). 



The fact that the temperature due to evaporation is often quite 

 different (sometimes amounting to fifteen degrees) from the tempera- 

 ture as indicated by the thermometer, is one of very great importance, 

 and one that is very commonly overlooked. 



