Kids Research Express

Chronometer, mechanical timekeeping device of great accuracy, particularly one used for determining longitude (see latitude and longitude ) at sea. Early weight- and pendulum-driven clocks were inaccurate because of friction and temperature changes and could not be used at sea because of the ship's motion. In 1735 John Harrison invented and constructed the first of four practical marine timekeepers. The modern marine chronometer is suspended to remain horizontal whatever the inclination of the ship and differs in parts of its mechanism from the ordinary watch. A chronometer may provide timekeeping accurate to within 0.1 second per day. See also Ferdinand Berthoud.

Electrocardiography (ĭlĕk'trōkärdēŏg`rəfē), science of recording and interpreting the electrical activity that precedes and is a measure of the action of heart muscles. Since 1887, when Augustus Waller demonstrated the possibility of measuring such action, physicians and physiologists have recorded it in order to study the heart's normal behavior and to provide a method for diagnosing abnormalities. Electrical current associated with contraction of the heart muscles passes through the various tissues and reaches the surface of the body. What is actually recorded is the change in electrical potential on the body surface. The first practical device for recording the activity of the heart was the string galvanometer developed by William Einthoven in 1903. In this device a fine quartz string is suspended vertically between the poles of a magnet. The string is deflected in response to changes in electrical potential and its movement can be optically enlarged and photographed, or, if

Nutation, in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes , involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.6 years, the same as that of the precession of the moon's nodes, as opposed to the nearly 26,000-year period of the precession of the equinoxes. Nutation was discovered by the English astronomer James Bradley in 1728 but was not explained until 20 years later.

Some elements have been known since antiquity. Gold ornaments from the Neolithic period have been discovered. Gold, iron, copper, lead, silver, and tin were used in Egypt and Mesopotamia before 3000 B.C. However, recognition of these metals as chemical elements did not occur until modern times. Greek Concept of the Elements The Greek philosophers proposed that there are basic substances from which all things are made. Empedocles proposed four basic "roots," earth, air, fire, and water, and two forces, harmony and discord, joining and separating them. Plato called the roots stoicheia (elements). He thought that they assume geometric forms and are made up of some more basic but undefined matter. A different theory, that of Leucippus and his followers, held that all matter is made up of tiny indivisible particles (atomos). This theory was rejected by Aristotle, who expanded on Plato's theory. Aristotle believed that different forms (eidos) were assumed by a basic material

Spectroscope, optical instrument for producing spectral lines and measuring their wavelengths and intensities, used in spectral analysis (see spectrum ). When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material. In the original spectroscope design in the early 19th cent., light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. A prism then separated the beam into its spectrum. The observer then viewed the spectrum through a tube with a scale that was transposed up the spectrum image, enabling its direct measurement. With the development of photographic film, the more accurate spectrograph was developed. It was based on the same principle as the spectroscope, but it had a camera in place of the telescope. In recent years the electronic circuits built around the photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis of far greater accuracy. Such spe

In the N and W United States the era of mechanized agriculture began with the invention of such farm machines as the reaper, the cultivator, the thresher, and the combine. Other revolutionary innovations, e.g., the tractor, continued to appear over the years, leading to a new type of large-scale agriculture. Modern science has also revolutionized food processing; refrigeration, for example, has made possible the large meatpacking plants and shipment and packaging of perishable foods. Urbanization has fostered the specialties of market gardening and truck farming. Harvesting operations (see harvester) have been mechanized for almost every plant product grown. Breeding programs have developed highly specialized animal, plant, and poultry varieties, thus increasing production efficiency. The development of genetic engineering has given rise to genetically modified transgenic crops and, to a lesser degree, livestock that possess a gene from an unrelated species that confers a desired quali

As the Middle Ages waned, increasing communications, the commercial revolution, and the rise of cities in Western Europe tended to turn agriculture away from subsistence farming toward the growing of crops for sale outside the community (commercial agriculture). In Britain the practice of inclosure allowed landlords to set aside plots of land, formerly subject to common rights, for intensive cropping or fenced pasturage, leading to efficient production of single crops. In the 16th and 17th cent. horticulture was greatly developed and contributed to the so-called agricultural revolution. Exploration and intercontinental trade, as well as scientific investigation, led to the development of horticultural knowledge of various crops and the exchange of farming methods and products, such as the potato, which was introduced from America along with beans and corn (maize) and became almost as common in N Europe as rice is in SE Asia. The appearance of mechanical devices such as the sugar mill