When Touchstone said, so very wisely, "It is ten o'clock," he used a phrase which, according to Orlando in the same play, could only properly apply to a mechanical time-piece. Rosalind asks Orlando, "I pray you what is it a clock?" to which he replies, "You should ask me what time o' day; there's no clock in the forest." Again, when Jacques declares that he did laugh "an hour by his dial," do we not immediately recall Falstaff's similar phrase, "an hour by Shrewsbury clock?"
If it shall be said that the word "dial" is more used in reference to a natural than to a mechanical indicator of time, I should point, in reply, to Hotspur's allusion:
"Tho' life did ride upon a dial's point
Still ending with the arrival of an hour"
The "dial's point," so referred to, must be in motion, and is therefore the hand or pointer of a mechanical clock.
A further confirmation that the Shakspearian "dial" was a piece of mechanism may be seen in Lafeu's reply to Bertram, when he exclaims,
"Then my dial goes not true,"
using it as a metaphor to imply that his judgment must have been deceived.
These are some of the considerations that would induce me to reject Mr. Knight's interpretation, and, were it necessary to realize the scene between Jacques and Touchstone at all, I should prefer doing so by imagining some old turnip-faced atrocity in clock-making presented to the fool's lack-lustre eye, than the nice astronomical observation supposed by Mr. Knight.
The ring-dial, as described by him, and by your correspondents, is likewise described in most of the encyclopædias. It is available for the latitude of construction only, and was no doubt common enough a hundred years ago; but it is scarcely an object as yet for deposit in the British Museum.