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nature should be assigned, on account of the frequent and 

 usual concurrence of several natures, the instances of the 

 cross show that the union of one nature with the required 

 nature is firm and indissoluble, while that of the other is 

 unsteady and separable; by which means the question is 

 decided, and the first is received as the cause, while the 

 other is dismissed and rejected. Such instances, therefore, 

 afford great light, and are of great weight, so that the course 

 of interpretation sometimes terminates, and is completed in 

 them. Sometimes, however, they are found among the 

 instances already observed, but they are generally new, 

 being expressly and purposely sought for and applied, and 

 brought to light only by attentive and active diligence. 



For example: let the required nature be the flow and 

 ebb of the sea, which is repeated twice a day, at intervals 

 of six hours between each advance and retreat, with some 

 little difference, agreeing with the motion of the moon. We 

 have here the following crossways: 



This motion must be occasioned either by the advancing 

 and the retiring of the se.a, like water shaken in a basin, 

 which leaves one side while it washes the other; or by the ris 

 ing of the sea from the bottom, and its again subsiding, like 

 boiling water. But a doubt arises, to which of these causes 

 we should assign the flow and ebb. If the first assertion be 

 admitted, it follows, that when there is a flood on one side, 

 there must at the same time be an ebb on another, and the 

 question therefore is reduced to this. Now Acosta, and 

 some others, after a diligent inquiry, have observed that 

 the flood tide takes place on the coast of Florida, and the 

 opposite coasts of Spain and Africa, at the same time, as 

 does also the ebb; and that there is not, on the contrary, a 

 flood tide at Florida when there is an ebb on the coasts of 



SCIENCE Vol. 229 



