148 EFFECTS OF TIDAL ACTION. [October, 



Such an operation of Nature at the beach, and par- 

 ticularly in our situation, would inevitably destroy the 

 Observatory, perhaps leave no hope of saving the instru- 

 ments ; but this I did not anticipate now. The bay or 

 land ice had formed, and the planes for this season would 

 not in all probability be disturbed to any extent ; but it 

 was to be feared. Another cause in daily operation, and 

 acting to the same end, was in quiet, irresistible action : 

 this was the tide. If the elder Perkins is alive, he may 

 remember explaining to me his earliest invention, the 

 cotton-pressing machine ; I think it may still be seen at 

 the Polytechnic Institution. Such is our tidal ice-power, 

 the "taking up wedges" being the snow-drift, etc., which 

 fills in as the tide opens the various fissures following 

 the contour of the coast margin. 



Let it be conceded that a great and continuous space 

 at high water is frozen, and that the established winter 

 ice, grounding daily in sixteen feet water, causes gaps 

 varying from one foot to six inches, in contour hues cor- 

 responding to the depths surrounding the land. It must 

 be evident that these gaps, if filled up, must at each re- 

 turning high water (twice daily) force the in-shore ice 

 tip the inclined plane equal in amount to the interposed 

 compressed matter. This we found it did, until the 

 heavy ice formed, to guard the in-shore line which it had 

 taken up. Then these gaps presented another feature : 

 they turned up, or rose above their in-shore grounded 

 pieces, causing, wherever resistance offered, unpleasant 

 barriers. The upper crack of the ice-line ceased about 

 six feet from the Observatory steps. I cannot state 

 precisely how many yards it was from high-water line 



