Abstract
In a communication to the Royal Irish Academy (Proceedings of the Royal Irish Academy, December, 1875), in which I examined the experiments made by M. Dohrandt to determine the constants of the cup anemometer by means of Robins’ whirling machine, I expressed a hope that I might have an opportunity of making similar experiments free from some influence which seemed to me objectionable. This opportunity was soon afforded me by a liberal grant from the Donation Fund of the Royal Society. I lost no time in having the necessary apparatus constructed by Mr. Howard Grubb, of Dublin, and I now propose to give the results which I have obtained with it. (1.) I was at first embarrassed by the difficulty of finding a suitable locality for the machine. In the St. Petersburg experiments the anemometers passed in their rotation at unequal distances from the walls of the building, and were too near its roof, which seemed likely to introduce extra disturbances into the air vortex which is formed by the whirl of the machine, and which, as will be seen, is a notable element of uncertainty in these investigations. But this was removed by Mr. H. Grubb placing at my disposal the central dome of his works at Rathmines, then recently constructed for mounting the great equatorial which he is making for the Vienna Observatory. Its basis is a regular dodecagon, 42 feet least diameter, and 42 feet from its floor to the summit of the dome. Round it, at 10 feet from the ground, runs a gallery 2 feet wide, with a light iron railing, from which several doors give access to workshops in which polishing and other light work is executed. On the ground, open arches communicate with shops, in which large lathes and other heavy tools are established. This seemed all that I could wish, but, unfortunately, I could have the use of it only till the end of June, when it was wanted for the erection of the equatorial. There were, however, two inconveniences to be feared. One was that the open arches might admit irregular currents of air ; the other was that when heavy turning or planing was going on, tremors were produced in the ground which might be expected to make the friction of the apparatus vary. I supposed, however, that this disturbance would equally affect the motion of the anemometers and the measures of their frictions ; but this influence proved to be of far greater importance than I had anticipated. On the other hand, the vicinity of skilled workmen, ready to make any necessary repairs or any alteration suggested by my experiments, was of great importance, and not less so the help which Mr. Grubb and his assistant, Mr. Vereker, gave me during the whole of this work.