424 The Account of a 
Arbour, where tents were pitched for a party of the royal re- 
giment of artillery, consisting of one serjeant and ten gunners, 
who were to be employed in the laborious part of the ope- 
ration. 
art. in. 'Experiments made to ascertain the relative Lengths of 
the Chains, before and after they were used ; and also to de- 
termine the Expansion of one Chain, or one hundred Feet of 
blistered Steel, by one Degree of Fahrenheit's Thermometer . 
For this purpose, two strong oaken pickets were driven two 
feet into very firm ground, and the drawing-post was made 
fast to them. Five coffers' were arranged in a right line, and 
supported upon courses of bricks. The chain was then placed 
in the coffers, and stretched with a weight of fifty-six pounds. 
Notwithstanding the great resistance which it w r as thought 
these pickets were capable of, yet it was found insufficient to 
counteract the friction between the coffers and the chain, 
when the expansion or contraction took place. Three pickets, 
therefore, of forty-four inches long, were driven into the 
ground, within six inches of their tops, and the drawing-post 
was fastened to them by several folds of strong rope. The 
pickets and rope were also covered with earth, to prevent their 
being warped by the sun. 
The micrometer-screw, attached to the brass register-head, 
by means of which the expansion or contraction was measured, 
contains 26 threads in an inch. The circular head is divided 
into 10 equal parts, and consequently each division will mea- 
sure P art °f an inch. But as the eye readily subdivides 
