It is important to notice that the expansion of paper is different for the two directions, that is for the machine and cross directions.
This arises from the fact that in the machine-made paper the greater proportion of the fibres point in the direction of the machine while the paper is being made. In consequence of this the expansion of the paper is greatest in what is known as the cross direction of the paper, that is, in the direction at right angles to the flow of the pulp along the machine wire.
This is to be explained by reference to the behaviour of fibres when damped or brought into contact with an excess of water. The question of the exact changes in the dimensions of a fibre due to absorption of water has been dealt with in an interesting manner by Hohnel. He points out that the well-known peculiarity of the shrinkage of ropes which have been lying in the water can be explained by an examination of the behaviour of the single fibres. He relates in detail the experiment which can be carried out for the exact observation of the fibres when in contact with water. A dry fibre when soaked in water appears to become 20 to 30 per cent. greater in diameter, whereas in length it is usually only increased by one-tenth per cent.
The method adopted by Hohnel was to place a fibre of convenient length on a glass slip down the centre of which was a fine narrow groove capable of holding water, so that the fibre could be wetted. Over the fibre was a cover glass with a small scale marked on it. The loose end of the fibres passed over a small roller and was stretched by a light weight. The movements of the fibre were measured by means of an eye-piece micrometer.
In this way it is possible to determine alterations in length to within 0·005 per cent., and this variation can be directly seen under the microscope.
Hohnel observes in his account of the experiments that all fibres become thicker when wetted, that vegetable fibres are more susceptible than animal fibres.
Animal fibres expand about 10 to 14 per cent. in diameter, but vegetable fibres as much as 20 per cent., as shown in the following table:—
| Animal Fibre. | Per Cent. | Vegetable Fibre. | Per Cent. |
| Human hair | 10·67 | New Zealand flax | 20·0 |
| Angora wool | 10·2 | Aloe hemp | 25·8 |
| Alpaca wool | 13·7 | Hemp | 22·7 |
| Tussah silk | 11·0 | Cotton | 27·5 |
The reverse is the case when the expansion of the fibres in regard to length is considered, since animal fibres expand 0·50 to 1·00 per cent. of their length, and vegetable fibres only 0·05 to 0·10 per cent.