The Editor,

The Journal of Glaciology

Sir,

I am most grateful to Professor Evers for pointing out the references to his important papers on this subject which had indeed escaped my notice.

Our measurements in Lyngen were taken over such a short period of time that the problem of ablation scarcely arose, but this could become very serious over longer periods as Professor Evers points out. However, its effects can be minimized by mounting the apparatus on tubes driven deep into the glacier and filled with a freezing mixture of ice and salt. Over a considerable period of time the pull of the wires would undoubtedly cause a movement of the instrument towards the valley side, thus introducing an error, but this can be minimized in the same way as the effect of ablation. On a narrow glacier it should be possible to have two instruments on the same mounting, but with wires running to opposite sides of the valley so that this error would then be entirely eliminated. It is true, however, that in its present form the instrument cannot record lateral movements which may be very important. Indeed periods of apparent reduction in forward speed of the glacier as recorded by the instrument may in fact correspond to periods of intensified lateral or upward motion. Very much remains to be found out about glacier m otion and with further testing and development “glacier clocks” may become a source of much interesting information.