Dr Hibbert on the Dlstrihutkni 
head- way enough to bring them under command of the rudder, 
but the Vulcan immediately springs off the bank, and takes 
into the middle of the canal 
Pumps for clearing away leakage or bilge water were put in- 
to her, but I believe they have hardly been required to be used. 
It has been under contemplation to cause the horses to track the 
boats through the intervention of a spring, in order to equalize 
their exertions, which vary, in very minute portions of time^ 
from 100 lb. to 1000 lb. while the average strain on the line is 
betAveen S50 lb. and 300 lb. It has been found impracticable, ’ 
hoAvever, to procure springs combining sufficient power of exten- 
sion and strength, and I believe we must have recourse to one 
whose temper will not be found liable to fail ; I mean that of air 
acted on by a piston in a barrel of adequate dimensions. Perhaps 
some of your readers will have the goodness to suggest a con- 
venient form. The points to be observed are, that the appara- 
tus do not incommode the passengers, and that it be adapted 
to the use of such people as are employed in navigating the 
vessels. I remain, my dear Sir, very sincerely yours, 
John Robison. 
Art. tv. ^S ketch of the Distribution of Rocks in Shetland. 
By Samuel Hibbert, M. D. F. R. S. E. and M. W. S. &c. 
(Concluded from p. 79.) 
ROCES IN THE WESTERN DISTRICTS OF SHETLAND, COMPRISING 
THE PARISHES OF SANDSTING, WALLS, AND AITHSTING. 
The whole of these rocks are situated to the west of that 
extreme portion of the mass of epidotic sienite, which we may 
trace from Sand Voe, in a northerly direction, to Aith Voe. 
The rocks of this district may be thus enumerated : they are, 
1.9^, The granite of Sandsting ; ^dly^ The strata maintaining a 
• The form of the bottom of this boa^ was copied from a model sent by Ad- 
miral Shank. It excels the others which have been tried, as it occasions little 
agitation in the water, and is therefore favourable to the preservation of the Canal 
banks. 
I It is 'wished that the extension should not be less than three feet, with the 
force of 1000 lbs. 
