of other interfering causes which have a tendency to prevent desired results.  First, we will imagine a balance with its spring such as we find in fine marine chronometers.  It has small pivots running in highly-polished jewels; such pivots are perfectly cylindrical, and no larger than are absolutely necessary to endure the task imposed upon them—­of carrying the weight of the balance and endure careful handling.

To afford the necessary vibrations a spring is fitted, usually of a helical form, so disposed as to cause the balance to vibrate in arcs back and forth in equal time, provided these arcs are of equal extent.  It is now to be taken note of that we have it at our disposal and option to make these arcs equal in time duration, i.e., to make the long or short arcs the quickest or to synchronize them.  We can readily comprehend we have now established a very perfect measure of short intervals of time.  We can also see if we provide the means of maintaining these vibrations and counting them we should possess the means of counting the flights of time with great accuracy.  The conditions which surround our balance are very constant, the small pivots turning in fine hard jewels lubricated with an oil on which exposure to the action of the air has little effect, leaves but few influences which can interfere with the regular action of our balance.  We add to the influences an adjustable correction for the disturbances of heat and cold, and we are convinced that but little could be added.

ANTAGONISTIC INFLUENCES.

In this combination we have pitted two antagonistic forces against each other, viz., the elasticity of the spring and the weight and inertia of the balance; both forces are theoretically constant and should produce constant results.  The mechanical part of the problem is simply to afford these two forces perfect facilities to act on each other and compel each to realize its full effect.  We must also devise mechanical means to record the duration of each conflict, that is, the time length of each vibration.  Many years have been spent in experimenting to arrive at the best propositions to employ for the several parts to obtain the best practical results.  Consequently, in designing a chronometer escapement we must not only draw the parts to a certain form, but consider the quality and weight of material to employ.

To illustrate what we have just said, suppose, in drawing an escape wheel, we must not only delineate the proper angle for the acting face of the tooth, but must also take cognizance of the thickness of the tooth.  By thickness we mean the measurement of extent of the tooth in the direction of the axis of the escape wheel.  An escape-wheel tooth might be of the best form to act in conveying power to the balance and yet by being too thin soon wear or produce excessive friction.  How thick an escape wheel should be to produce best results, is one of the many matters settled only by actual workshop experience.