I think that is just saying to work smarter, not harder. I don't think it's saying put in fewer hours. The "as to be able to work constantly" seems to be vital here.
I provided the quote which I thought most concisely communicated his thesis in this passage. The immediately proceeding lines I believe clarify it:
"Great labour, either of mind or body, continued for several days together is, in most men, naturally followed by a great desire of relaxation, which, if not restrained by force, or by some strong necessity, is almost irresistible. It is the call of nature, which requires to be relieved by some indulgence, sometimes of ease only, but sometimes too of dissipation and diversion. If it is not complied with, the consequences are often dangerous and sometimes fatal, and such as almost always, sooner or later, bring on the peculiar infirmity of the trade. If masters would always listen to the dictates of reason and humanity, they have frequently occasion rather to moderate, than to animate the application of many of their workmen."
This is all from Chapter VIII: Of The Wages Of Labour.
