206 



NATURE 



[Dec. 27. 1SS8 



at -8'. In the icicle, when the temperature variations 

 seemed paramount, the rate at -2^ was o'oo28 ; and at 

 -o°-2, 0-00^4. Under pressure the influence of tempera- 

 ture seems much more powerful. In all three pieces 

 of glacier ice the rate rose at -3' to about ten times its 

 value at - 5'. 



The effect of a change of tension was very striking. 

 I append a list of all the cases which occurred. 



Specimen. 



Glacier ice C 

 Glacier ice D 



Change of rate : 

 rim. per hour per locm. 

 00018 to O'OIIO 



0-0075 ,, 0026 

 o-io5(,?) ,, o-oio 



O'OIO ,, 0'228 



1-88 ,, 0-35 



Change of tension 

 kilos per sq. cm. 



,2-55 to 3-85 



1-45 ., 2-55 



2-55 „ 1-03 



1-03 „ 250 



,, 2-50 ,, i-8o 



The o'io5 is uncertain owing to an accident. It was 

 certainly not less, and may have been a good deal 

 greater. 



I think it will be interesting to describe the system of 

 levers which we found so effective in measuring small 

 extensions. It is shown in the figure, a and b are 

 sections of the projecting ends of glass needles fixed in the 

 ice from 12 cm. to 20 cm. apart ; cdef is a bent iron wire, 

 ^'the indicator," hooked to a wire loop, w, securely fastened 

 to a; h is a wooden lever suspended by a thread n, which, 

 owing to the counterpoise /', pulls the indicator up- 

 wards with a thread fastened to a wire loop at e. The 

 indicator is kept from rising by the connecting fibre, a 

 piece of stiff wire hooked at one end to the loop g, 

 fastened to b, and at the other to a bend d^ in the indi- 

 cator. The lower end of the indicator gives the reading 

 on a paper millimetre scale /, gummed on to the mirror ;J. 

 The mirror, of course, enables the observer to avoid 

 errors of parallax. The stand of the mirror is glued to 

 the lower collar. To appreciate the action of the levers, 

 regard a for the moment as fixed, then lowering b through 

 a small distance ;- will move/ through a distance s = vr 

 at right angles to w/, where v is the ratio of the distance 

 mf to the perpendicular let fall from m on the line gd 

 produced if necessary. If 7nd be the perpendicular to gd, 

 when / is in the middle of the scale, the multiplier v 

 remains practically constant. This precaution was not 

 always taken, but allowance is made for the resulting 

 error. Two lever systems were required, one for the 



outer ends, and the other for the 

 needles passing through the ice. 



inner ends of the 



In the following table 

 results : — 



IS given a summary 



of our 



Summary. Extension Exftriments. 



Descrif tlon ot specimen. 



Bath ice uncorrected for temperature 



, , corrected for temperature 



Mould ice 



Glacier ice A, maximum rate 



,, minimum rate 



Glacier ice B, maximum rate 



Glacier ice C, ,, 



, , minimum rate 



Glacier ice D, maximum rate 



,, minimum rate 



,, lowest temperature 



Icicle, maximum rate 



,, minimum rate 



Lake ice, parallel columns 



,, greater tension 



Lake ice, oblique to columns \ "^^Z"^"'" ''^'^ - 

 ' '- I mmimum rate.. 



Rate per hour 



in mm. per length 



cf ic cm. 



5h days 



28 hours 

 5 ., 



4 „ 

 24 „ 

 23 ,, 



3 days 

 lomins. 

 16 hours 

 12 ,, 



5 ,, 

 8 „ 



7 days 

 2 ,, 



6 hours 

 16 ,, 



Comp-e. 



0-00028 



O'OCOOO 



0-048 

 0022 

 0013 



0016 



0-0068 



00013 



1-88 



0-0054 



0-0065 



0-0041 



0-0015 



o -00039 



0-00076 



0034 



O-OIO 



Experiments. 



Tension, kibs 

 per sq. cm. 



4-9 



38 



1 66 



2'-7 

 2-55 



2 50 

 1-45 



2-1 



2-8 

 2-75 



Maximum 

 temperature. 



00 

 00 

 10 



-2-5 



■25 

 -60 



Mean 

 temperature. 



4-5 



-60 

 -90 



o o 



-07 



0-0 



-40 



-5-6 



5-0.- 

 2-0 



■ 2-5 



3-5 



4"5 

 9-0 

 2-1 



ICO 



IO-5 

 00 

 17 



5'5 



■ 5-5 

 5-8 

 60 



This was a deeper herd than is shown in the figure. 



