TRANSACTIONS OF THE SECTIONS. 



131 



With respect to the nature of the experiments, a statement of a few principal 

 points will here suffice, as the form and construction of the apparatus cannot be 

 minutely explained in the absence of the drawings, which were exhibited in the Section 

 by the author. 



One of the chief difficulties anticipated by Mr. Thomson, in his attempts to find 

 good modes of experimenting, had reference to the determination of the quantity 

 of water lifted by the pump from the low level to the discharge level, and of that let 

 down from the high level to the discharge level. A very simple and effective mode of 

 obviating the difficulty was devised, as follows: — An apparatus was arranged, not for 

 ineasuring the absolute quantities of water lifted and let down in various experiments, 

 b'ut only the ratios of these quantities to one another ; and it is to be observed^ that, 

 for the purpose in view, precision in the ratios of the quantities, rather than in their 

 absolute amounts, was to be desired. 



A vessel or cistern was provided and set up at a level above the jet pump ; and all 

 the water to supply the pump, as well as all the water to be lifted by it, was made to 

 pass through the cistern, and to issue by a slit in its bottom, about one foot long, and 

 of a width which could be varied at pleasure, but was usually about one quarter of an 

 inch. The water thus issued in the form of a thin sheet one foot wide and about one 

 quarter of an inch thick descending vertically. Out of this sheet of water any portion 

 desired could be taken and conveyed away by means of a small moveable wedge-shaped 

 vessel made to slide in below the slit of the cistern. The water thus abstracted was 

 conveyed down to the low level to be lifted by the jet pump, and the remainder was 

 used for supplying the power in the jet pump. By observation of the width of the 

 portion of water abstracted from the sheet, and comparison of that with the width of 

 the whole sheet, the ratio of the two quantities was determined. 



The absolute quantities of water could be varied at pleasure, without any alteration 

 of their ratio, by increasing or diminishing the depth of water in the supply cistern 

 from which the sheet of water issued. Thus the absolute quantity was adjusted so as 

 to make either the high water supplying the power, or the low water lifted by the pump, 

 stand at any desired level while the pump was in continuous action. The one of those 

 two levels being thus fixed, the other, after some fluctuations, soon adjusted itself to 

 its permanent height, and the two permanent levels were then observed, and so one 

 experiment was completed. The series of experiments was made by successively 

 cutting out various portions of the sheet of water to be conveyed to the low level ; and 

 then observing the height of lift, and the height of fall, which corresponded to each 

 ratio of the quantity sent to be lifted, to the quantity sent to supply the power. 



The following table gives a summary of the chief practical results obtained. It is 

 derived frorn two sets of experiments made on a jet pump with a jet of seveTi-eighths of 

 an inch diameter. The height of fall varied from twenty-one inches to twenty-eight and 

 three-fourths inches, and the height of lift from six inches to thirty-six and a half 

 inches. 



On an Electric Semaphore for Use on Railways. By W. Sykes Ward. 



The object of the communication was, to show that a semaphore consisting of a 

 disc might be constructed to make a partial revolution, so as to take different positions 

 exhibiting three distinct signals, and that its motion might be regulated by electro- 

 magnets worked by a contiguous supplemental battery, of which the circuit is opened, 



9* 



