Page:Popular Science Monthly Volume 45.djvu/495

Rh at all—thus turning, as he expected, that island into an abode of arctic snow and ice.

Another feature of the case is the fact that the daily tides would not be the same on the two sides of the dam. To the north one could look clear out to sea over the Atlantic Ocean; to the south is about three hundred miles of practically inland water before one gets out to the open ocean coast. The tides on the open coast are about the same height and come at about the same times south and north; and at present, at any given point in the Irish. Sea, the height of the sea level at any time is determined by the resultant of the tides from the north and south respectively. The construction of a dam at the northern entrance would leave the whole Irish Sea subject only to the influence of the tides from the south, while on the north side of the dam the tide level would be the same as that of other points on the open coast. Since, now, it would take some time, probably several hours, for the effect of the southern tide to reach the south side of the dam, the tides on the two sides would be anything but synchronous. When the tide Was at its height at the north side it would be, perhaps, half-way up on the south, and would be high on the south by the time a considerable recession had taken place on the north. This variation would have a most important bearing on the working of the power machinery at the dam, because, instead of the difference of level between the water on the two sides being constant, and giving therefore a constant pressure, it would vary so as to be at times greater and at times less than would be the case if the effect alluded to did not take place. In order, therefore, to supply an equable driving head to the dynamos, the turbine wheels would have to be powerful enough to work up to the required capacity on the minimum difference of level. Since the power made available by the dam would be remarkable for the vast volume of water to be drawn on, rather than for great difference of level, the interference of the tides in at times reducing this difference perhaps considerably would be a matter of grave inconvenience in the way of the successful operating of the power generators.

system of school education, though judiciously criticised, is not regarded in the paper of Prof. Glynn, of Liverpool, on excessive mental work and some of its consequences, as being in a marked degree accountable for nervous overstrain in childhood. The tendency to this effect is considered to be in a great measure counteracted by the attention given to physical education and by the mental elasticity natural to youth. More serious are the consequences entailed by close and anxious application to duty of teachers and older students. As concerns the adult population, the injurious influence of overstrain is most active in towns, where the tension in the struggle for existence is greater and is associated with a desire too easily gratified.