1920.] Birks and Ferguson.—High-tension Insulators. 
181 
The effects of temperature-changes were taken into consideration, and 
during overhaul a careful inspection was made for cracked shells. No 
sign of cracking was observed during the first two years of operation, but 
in a later overhaul, after the fourth year, about thirty-six insulators were 
removed, cracked badly in the top shell. There seems little doubt that 
this cracking was due to unequal expansion and contraction of the porcelain 
und the cement, and possibly the wire clamp which clamps firmly about 
the neck of the insulator. 
The following test was carried out on four stock insulators which had 
been stored in their crates in the open for about two years. The insulators 
were plunged into boiling water and allowed to remain for a minute or two, 
