92 Mr. Alexander B. Binnie [March 6, 



vacuity, 4 inches in width, between the back of the plates and the 

 natural ground. This space is made solid by the injection of grout 

 under pneumatic pressure by means of the contrivance patented by 

 Mr. Greathead. This consists of a closed horizontal cylinder in 

 which lime or cement can be mixed to the consistency of thick cream 

 by a horizontal spindle with arms which pass through it. The upper 

 side is furnished with a pipe through which air pressure can be applied 

 to the surface of the grout, and from the lower side the grout is con- 

 veyed in another pipe to the holes in the plates through which it is 

 forced by the air pressure. 



To provide against accidents, two precautions are adopted in case 

 of an inrush of water : one is an elevated temporary wooden gangway 

 or path, extending from the shield to the upper escape air-lock in the 

 air-tight bulkhead, the other is a fixed curtain of wrought iron which 

 descends to the semi-diameter of the tunnel, so that in case of an 

 irruption of water it would not fill the entire tunnel, but a certain 

 portion of compressed air would be trapped between the curtain and 

 the air-lock, and so form a kind of elongated diving bell. 



Having now viewed all that is to be seen in compressed air, we 

 return to the air-lock for the purpose of passing out. This is in 

 some respects difterent from passing in, and is an operation requiring 

 some little time and caution, as the removal of the artificial air 

 pressure and the return to normal conditions is more than equivalent 

 to an ascent beyond the tops of the highest mountains on the earth, 

 as the artificial pressure may be 30 lbs. per square inch, all of which 

 has to be removed before we return to the normal 15 lbs. As to 

 sensation, no difficulties about the treatment of the ears is experienced 

 as the compressed air in the middle ear gradually and naturally dis- 

 charges itself with a not unpleasant crackling sound. Owing, however, 

 to the expansion of the air in the lock, the temperature falls rapidly, 

 so much so that the invisible aqueous vapour contained in the air is 

 deposited as a thick damp fog, and a chill is experienced ; beyond 

 this there is nothing particular to notice. From the tunnel we ascend 

 the shaft No. 4 and in the cabin at the top take a cup of hot coifee, 

 which slight stimulant is sufficient to restore the system to its usual 

 condition. 



I am frequently asked if we have found any objects of interest or 

 antiquity in our various excavations. But as most of our work has 

 been through the tertiary beds of the London clay and Woolwich 

 series, nothing but the fossils peculiar to these formations have been 

 met with. On the table will be found specimens of the base bed of 

 the London clay, and of the conglomerate bed which lies just below 

 it. These two formations have also been met with on other works, 

 as at Abbey Mills and the Beckton Gas-works. There is also a 

 specimen of the shelly clay of the Woolwich series. In the super- 

 ficial gravel, part of an elephant's tusk was found on the south shore 

 of the river; a similar tusk was also found on another work in the 

 gravel beds near Abbey Mills at Stratford. On the north side of the 



