The actual performance that can be obtained from a clock under the most favourable circumstances is a matter of considerable scientific as well as technical interest, and this has been stimulated recently by the development of the Shortt clock. While there is no question that this clock can hold its rate in a way that no other type will do, there is evidence that exaggerated expectations are entertained with respect to its performance, even in well-informed quarters. This is my reason for publishing the following particulars, which are a digested account of the going of two of these clocks at the Royal Observatory, Edinburgh, during the year 1927.