10 SURFACE GEOLOGY. 



ing other fragments for a time when its power should be increased. In this way 

 have the materials been sorted out more nicely than any mechanical skill could do. 



Details of the Facts. 



I now proceed to give an account of the facts which I have collected respecting 

 terraces and beaches, within the last six or seven years. I began their examina- 

 tion in 1849, and have since pursued it as diligently as my time and means would 

 allow. And having, during that period, traversed several of the countries of 

 Europe, I improved the opportunity to notice these phenomena, though it was out 

 of my power to make very numerous measurements. I have also travelled some- 

 what extensively in our own country, to complete the comparisons. But it is 

 along Connecticut river and its tributaries that I have made the most careful and 

 consecutive observation. After reading Mr. Chambers' Ancient Sea Margins, I felt 

 desirous of determining the true heights of the terraces in this valley, by mensura- 

 tion. For a time I used the common levelling instruments, and thus obtained 

 numerous sections. This method I found, however, to be so laborious, in a coun- 

 try like ours, where so few heights away from our railroads and canals have been 

 ascertained, that some other method would be important, where the beach or ter- 

 race to be measured was distant from any such ascertained heights. I obtained an 

 Aneroid Barometer ; but my early trials with it were so unsatisfactory that I gave 

 it up in despair. But when I reached Liverpool, and was desirous of visiting the 

 mountains of Wales, I purchased another, and found the results so satisfactory in 

 the measurement of Snowdon and Cader Idris, that I carried it with me in all my 

 wanderings. In going to Ireland, however, the hair-spring that regulates the 

 index, was broken by the rough usage of my luggage. It was mended in Edin- 

 burgh, but broken again before I reached Frankfort on the Main. Again I had it 

 mended, and made use of it in Switzerland and Savoy. On my return to this 

 country, I wished to ascertain whether the accidents to which it had been subject 

 had affected its range. I soon discovered that they had. But instead of attempt- 

 ing to use the adjusting screws, I obtained from the Smithsonian Institution the 

 loan of one of Green's Syphon Barometers, and commenced a series of observations 

 in connection with the Aneroid. Those were at length reduced, and thus making 

 the Syphon Barometer the standard, I ascertained the error of the Aneroid, and 

 found that for every tenth of an inch it gave only 78.47 feet of altitude. Thus 

 was I able to correct all my observations made in Europe, after the injury of the 

 instrument, and the results I shall give below. 



Having used the Aneroid Barometer so extensively, it might be desirable that I 

 should go into details respecting the results, as compared with other measurements, 

 in order to decide how much dependence can be placed upon the instrument. But 

 these details would occupy too much space. If in my power, I hope to present 

 them in some other form : for my own conviction is, that though this instrument 

 cannot be depended on for nice observations, such as the mathematician needs, yet 

 it is a most valuable help to the geologist. I think it can be depended on almost 

 as confidently as the Syphon Barometer, except perhaps for very great altitudes. 



