The 19th century saw a massive surge in the development of postal services in the United States. The population of the country increased tenfold in the period from 1790 to 1860, and the postal offices and employee numbers reflected this growth – the number of post offices increased from 75 to over 28,000 in this period. New methods of mail delivery were pioneered, such as steamboats and railroad, but quicker service to the Pacific Coast was needed. This led to the development of the iconic Pony Express – a system using hardy horses, good riders, and relay stations 10 – 15 miles apart to ensure fresh mounts for the long journeys. Although the system only ran for 18 months, it remains an iconic symbol of American postal history. At a time when air travel was just an experiment, the Post Office was instrumental in investigating the possibilities of mail delivery via air. Their efforts paid off, and by 1918, Congress approved a large sum to be devoted to providing airmail routes and the system has gone from strength to strength since.