The
fibres of the wood you know are dilated, but not lengthened by moisture.
The slip, therefore, whose grain is lengthwise, becomes a standard,
retaining always the same precise length. That which has its grain
crosswise, dilates with moisture, and contracts for the want of it.
If the right hand piece be the cross-grained one, when the air is very
moist, it lengthens, and forces its companion to form a kind of interior
annulus of a circle on the left. When the air is dry, it contracts,
draws its companion to the right, and becomes itself the interior
annulus. In order to show this dilation and contraction, an index is
fixed on the upper end of the two slips; a plate of metal or wood is
fastened to the front of the plinth, so as to cover the two slips from
the eye. A slit, being nearly the portion of a circle, is cut in this
plate, so that the shank of the index may play freely through its whole
range. On the edge of the slit is a graduation. The objection to this
instrument is, that it is not fit for comparative observations, because
no two pieces of wood being of the same texture exactly, no two will
yield exactly alike to the same agent.
Pages:
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165