386 Proceedings of the Royal PJujsical Society. 



whether the motor be going fast or slow, the pump does its 

 work in proportion. 



Perhaps a comparison of the cost of working both pumps 

 would prove interesting. I find, on reference to my diary, 

 that from 1st September 1883 to 1st February 1884, while 

 the old pump was working, 132,540 gallons of fresh water 

 were used by the motor, or an average of 26,508 gallons per 

 month, which, at 6d. per 1000 gallons, gives an average 

 monthly cost of 13s. 3d. for water. This, indeed, is below 

 the average cost previously, for I see from a note in the 

 diary that the water meter was disconnected for repairs for 

 several days in January, so that during that time the water 

 used would not be registered. From September to December 

 1883 the monthly average was 28,000 gallons, which would 

 bring up the monthly cost to 14s. From 1st March 1884 to 

 1st January 1885, when the new pump only was used, 

 145,630 gallons of water were registered by the meter, giving 

 a monthly average of 14,500 gallons, and representing a 

 monthly cost of 7s. 3d. Thus, to begin with, the new pump 

 costs roughly only half as much as the old one to work. 

 This cost has been very much further reduced by means of 

 improved jets. 



Improvements in the Construction of Jets. 

 It appeared to me at a very early stage in my inquiry, 

 that the vulcanite jets, as arranged in public aquaria, do their 

 work very inefficiently. Unless we have facilities for chang- 

 ing the whole body of water in a tank very frequently by 

 means of a strong stream, it is necessary to introduce air into 

 the water by some mechanical means, so that the oxygen it 

 contains may be absorbed. For this purpose the smaller the 

 bubbles of air, the more likely they are to be of service, 

 because they require longer to rise again to the surface. 

 Thus the water has a better chance of absorbing the oxygen 

 they contain. Now, in the first place, the size of the orifice, 

 and in the second, the height of the jet above the water, 

 debars the ordinary vulcanite arrangement from bringing 

 about these conditions. The height of the jet above the 

 water necessitates a large orifice, or very few air bubbles will 



