VII] BEDFORDSHIRE: SURVEYING 109 



information that there was such a science as geology, and 

 that chalk was not everyzvhere found under the surface, as I 

 had hitherto supposed. My brother, like most land-surveyors, 

 was something of a geologist, and he showed me the fossil 

 oysters of the genus Gryphaea and the Belemnites, which we 

 had hitherto called "thunderbolts," and several other fossils 

 which were abundant in the chalk and gravel around Barton. 

 While here I acquired the rudiments of surveying and map- 

 ping, as well as calculating areas on the map by the rules of 

 trigonometry. This I found very interesting work, and it 

 was rendered more so by a large volume belonging to my 

 brother giving an account of the great Trigonometrical Survey 

 of England, with all the angles and the calculated lengths of 

 the sides of the triangles formed by the different stations on 

 hilltops, and by the various church spires and other con- 

 spicuous objects. The church spires of Barton and Higham 

 Gobion had been thus used, and the distance between them 

 accurately given ; and as the line from one to the other ran 

 diagonally across the middle of the parish we were survey- 

 ing, this was made our chief base-line, and the distance as 

 measured found to agree very closely with that given in the 

 survey. This volume was eagerly read by me, as it gave an 

 account of all the instruments used, including the great theo- 

 dolite three feet in diameter for measuring the angles of the 

 larger triangles formed by distant mountain tops often twenty 

 or thirty miles apart, and in a few cases more than a hundred 

 miles ; the accurate measurement of the base-lines by steel 

 chains laid in wooden troughs, and carefully tightened by 

 exactly the same weight passing over a pulley, while the ends 

 were adjusted by means of microscopes ; the exact tempera- 

 ture being also taken by several thermometers in order to allow 

 for contraction or expansion of the chains ; and by all these 

 refinements several base-lines of seven or eight miles in length 

 were measured with extreme accuracy in distant parts of the 

 country. These base-lines were tested by repeated measure- 

 ments in opposite directions, which were found to differ only 

 by about an inch, so that the mean of all the measurements 

 was probably correct to less than half that amount. 



