NIAGARA 207 



But this was not all. By protecting certain portions 

 of tlie surface, and exposing others, figures and tracery of 

 any required form could be etched upon the glass. The 

 figures of open iron- work could be thus copied; while 

 wire-gauze placed oyer the glass produced a reticulated 

 pattern. But it required no such resisting substance as 

 iron to shelter the glass. The patterns of the finest lace 

 could be thus reproduced; the delicate filaments of the 

 lace itself offering a sufficient protection. All these 

 effects have been obtained with a simple model of the 

 sand-blast devised by my assistant. A fraction of a 

 minute suffices to etch upon glass a rich and beautiful 

 lace pattern. Any yielding substance may be employed 

 to protect the glass. By diffusing the shock of the par- 

 ticle, such substances practically destroy the local erosive 

 power. The hand can bear, without inconvenience, a 

 sand- shower which would pulverize glass. Etchings exe- 

 cuted on glass with suitable kinds of ink are accurately 

 worked out by the sand-blast. In fact, within certain 

 limits, the harder the surface, the greater is the concen- 

 tration of the shock, and the more effectual is the erosion. 

 It is not necessary that the sand should be the harder 

 substance of the two; corundum, for example, is much 

 harder than quartz; still, quartz-sand can not only depol- 

 ish, but actually blow a hole through a plate of corundum. 

 Nay, glass may be depolished by the impact of fine shot; 

 the grains in this case bruising the glass before they have 

 time to flatten and turn their energy into heat. 



And here, in passing, we may tie together one or two 

 apparently unrelated facts. Supposing you turn on, at 

 the lower part of a house, a cock which is fed by a pipe 

 from a cistern at the top of the house, the column of 



