y CLOUDS AND RIVERS, ICE AND G& 



"Travels in tho Alps." 



154. These observations were also followed 

 up in subsequent years, the results being re- 

 corded in a seties of detached letters and es- 

 says of great interest. These were subse- 

 quently collected in a volume entitled " Oc- 

 casional Papers on the Theory of Glaciers," 

 published in 1859. The labors of Agassiz 

 and Forbes are the two chief sources of our 

 knowledge of glacier phenomena. 



21. THE THEODOLITE AND ITS USE. Oui 

 N MEASUREMENTS. 



155. My object thus far is attained. i 

 have given j r ou proofs of glacier motion, and 

 a historic account of its measurement. And 

 now we must try to add a little to the knowl- 

 edge of glaciers by our own hUws on the 

 ice. Resolution must not be wanting at the 

 commencement of our work, ^or steadfast 

 patience during its prosecution. Look then 

 ac this theodolite ; it consists mainly of a 

 telescope and a graduated circle, the tele- 

 scope capable of motion up an( i down, and 

 the circle, carrying the telescope along with 

 it, capable of motion right and left. When 

 desired to make the motion exceedingly fine 

 an'-l minute, suitable screws, called tangent 

 screws, are employed. The instrument is 

 supported by three legs, movable, but firm 

 when properly planted. 



150. Two spirit-levels are fixed at right 

 angles to each other on the circle just refei- 

 red to. Practice enables one to lake hold of 

 the legs of the instrument, and so to fix them 

 that tbe circle shall be nearly horizontal. By 

 means of four levelling screws we render it 

 accurately horizontal. Exactly under the 

 centre of the instrument is a small hook from 

 which a plummet is suspended ; the point of 

 the bob just touches a rock on which we 

 make a mark ; or if the earth be soft under- 

 neath, we drive a stake into it exactly under 

 the plummet. By re-suspending the plum- 

 met at any future time we can find to a hat"- 

 breadth the position occupied by the instru- 

 ment to-day. 



157. Look through the telescope ; you see 

 it crossed by two fibres of the finest spider's 

 (bread. In actual work we first direct the 

 telescope across the glacier, until the inter- 

 section of the two fibres accurately covers 

 some well-defined pcint of rock or tree at the 

 other side of the valley. This, our fixed 

 standard, we sketch with its surroundings in 

 ft note-book, so as to be able immediately to 

 recognize it oa our return to this place. Irn- 

 iginc a straight line drawn from the centre 

 of the telescope to this point, and that this 

 line in permitted to drop straight down upon 

 the glacier, every point of it falling as a 

 stone would fall ; along such a line we have 

 now to fix a series of stakes. 



158. A trained assistant is already upon 

 the glacier. He erects his staff and stands 

 behind it ; the telescope is lowered without 

 swerving to the right or to the left ; in mathe- 

 matical language it remains in tlie xame zerti- 

 "Mi plane. " The crossed fibres of the tele- 

 scope probably strike the ice a little away 



from the staff of the assistant ; by a wave of 

 the arm he moves right or left ; he may 

 move too much, so we wave him back again. 

 After a trial or two be knows whether he is 

 near the proper point, and if so makes his 

 motions small. He soon exactly strikes the 

 point covered by the intersection of the 

 fibres. A signal is made which tells him 

 that he is right ; he pierces the ice with an 

 auger and drives in a stake. He then goes 

 forward, and in precisely the same manner 

 takes up another point. After one or two 

 stakes have been driven in, the assistant is 

 able to take up the other points very rap- 

 idly. Any requisite number of stakes may 

 thus be fixed in a straight line across tht> 

 glacier. 



159. Next morning we measure the motion 

 of all the stakes. The theodolite is mounted 

 in its former position and carefully levelled 

 The telescope is directed first upon the 

 standard point at the opposite side of the 

 valley, being moved by a tangent screw until 

 the intersection of the spider's threads accu- 

 rately covers the point. The telescope is 

 then lowered to the first stake, beside which 

 our trained* assistant is already standing. 

 He is provided with a staff with feet and 

 inches marked on it. A glance shows us 

 that the stake has moved down. By our sig- 

 nals the assistant recovers the point from 

 which we started yesterday, and then deter- 

 mines the distance from this point to the 

 stake. It is, say, inches ; through this dis- 

 tance, therefore, the stake has moved. 



160. We are careful to note the hour anfl 

 minute at which each stake is driven in, and 

 the hour and the minute when its distance 

 from its first position is measured ; this ena- 

 bles us to calculate the accurate daily motion 

 of the point in question. The distances 

 through which all the other points have 

 moved are determined in precisely the same 

 way. 



161. Thus we shall proceed to work, first 

 making clear to our minds what is to be 

 done, and then making sure that it shall be 

 accurately done. To give our work reality, 

 I will here record the actual measurements 

 executed, and the actual thought suggested, 

 on the Mer de Glace in 1857. The only un- 

 reality that I would ask you to allow, Is that 

 you and I are supposed to be making the ob- 

 servations together. The labor of measuring 

 was undertaken for the most part by Mr. 

 Hirst. 



22. MOTION OP THE MER DE GLACE. 



162. On July 14, then, we find ourselves 

 at the end of the Glacier des Bois, not far 

 from the source of the Arveiron. We direct 

 our telescope across the glacier, and fix the 

 intersection of its spider's threads accurately 

 upon the edge of a pinnacle of ice. We 

 leave the instrument untouched, looking 

 through it from hour to hour. The edge of 

 ice moves slowly, but plainly, past the fibres, 

 and at the end of three hours we assure our- 

 selves that the motion has amounted to sev- 

 eral inches. While standing near the vault 



