364 Mr A. J. Apix on the Expansion of Stone 
of the instruments for similar operations ; but Lieutenant Mur- 
puy informed me, that he thinks it was employed in some py- 
rometric operations connected with a trigonometrical survey in 
India; and I am satisfied, from the uniformity of temperature it 
maintains in the instrument, no better method of heating can be 
resorted to for such operations, because the substance, if of 
wood or stone, is kept dry, and the fire or lamps are placed at such 
a distance as not to incommode the operator in the least degree. 
Some of the rods of stone experimented on are now before the 
Society. I had considerable difficulty in making rods of Roman 
cement, as they repeatedly cracked in drying, and I was anxious 
to have this substance tried, as I found that rods of lime were 
too weak, not more to settle the general question, as to the ex- 
pansion of buildings, than to have the pleasure, if possible, of as- 
sisting Mr Brunet in accounting for the results of some experi- 
ments he mentioned at the last meeting of the British Asso- 
ciation, when describing one of the many beautiful contrivances 
for which his fertile ingenuity has rendered him so celebrated. I 
succeeded in making a rod of Roman cement, by at first mixing 
as little water with the powder as would make it work, and when 
it had become too plastic, from continued working, more of the 
powder was added, and the cement again made plastic by work- 
ing and beating. This process was continued so long as any of 
the cement in powder could be added to the mass, and the whole 
again worked into the consistency of soft putty, without any ad- 
dition of water, as the strength of the mortar appears to de- 
pend on the smallness of the quantity of water with which we 
can make the dry powder plastic, this is accomplished by long 
continued working. After preparing the mortar, it was put into a 
mould of Bristol board, and when it had set, it was immersed in 
water, and left for a fortnight to harden. 
Having ascertained that all the parts of the pyrometer worked 
well from the uniformity of the results I obtained in determin- 
ing the expansion of cast zinc, which was selected as the metal 
