Page:Popular Science Monthly Volume 84.djvu/114

110 Pickering has now been for about thirty-seven years the head of the astronomical observatory at Harvard. The present physical laboratory at Harvard was built in 1884. Of late years laboratories have been built at all our colleges, and there has developed a tendency to make them very large and costly. Two of the latest, the Palmer Laboratory at Princeton and the Sloane Laboratory at Yale, have gone well beyond the quarter of a million mark. The largest and best equipped laboratory in the country is that belonging to the national government, and known as the Bureau of Standards, which, in its brief history of about ten years, has had over two millions of dollars spent upon it. When we consider that this institution is entirely separated from teaching, we must believe that work of great importance is done there to justify this great outlay. Permit me to describe what some of the functions of such a laboratory are, and incidentally to explain some of these devices that are to be found in any great modern laboratory.

At the Bureau of Standards we find five large buildings, each devoted to a particular purpose. These have cost $712,000. In the largest we find the divisions of weights and measures, of heat, and of light. The chief objects of the Bureau of Standards being necessarily practical, the researches undertaken there are limited in scope by this consideration. Nothing is perhaps more practical than the verification of the standards of weight by which commodities are bought and sold. Even the weights of the mint are tested at the Bureau of Standards. Accordingly in a basement room mounted upon heavy brick piers, which are a prominent feature in every physical laboratory in order to secure freedom from vibration, we find extremely accurate balances, some of which are capable of weighing a body with an accuracy of one part in fifteen or twenty millions. The comparison of two equal weights is probably susceptible of greater accuracy than any other physical operation. It is to be remarked that in order to attain this degree of accuracy the balance has to be operated in vacuo, the whole instrument being placed in a case from which the air is pumped out, and all operations of transferring the weights being conducted from a distance by means of controlling rods or shafts, since the heat of the observer's body near the balance would so change the length of the beam as to render such an accuracy impossible.

Next to weighing comes the measurement of length, which is susceptible of about the same accuracy. Here again, the effect of changes of temperature in causing metal scales to expand has to be provided against, so that the work has to be carried on in a subterranean vault, where the changes of temperature are made as small as possible. In the division of heat great practical importance belongs to the measurement of temperatures. Thousands of thermometers of all sorts are sent here yearly to be tested. Before the existence of the Bureau of Standards