Since Charles Babbage’s Analytical Engine wasn’t actually built until the 1940’s, many computing needs were met with the invention of the analog computer during the late 19th century and first half of the 20th century. Analog computers were computers that operated with numbers represented by measurable quantities, using either electronic or mechanical models of the problem in order to solve it. Analog computers look nothing like the PCs today as they operate much differently from digital computers and were huge, taking the size of a room. 
The analog system is a system of proportions, where one can see it as values of percentages. The answers to a problem were obtained by measuring the different variables in the model of the problem. The system only operated on continuous values such as voltage, temperature, pressure, etc., as opposed to the digital system which uses discrete values.
The earliest analog computer invented was the tide-predicting machine by Sir William Thomson in 1872. The tide-predicting machine did exactly what it says in the name, predicting the flow of sea tides and their variations in height. Four years later in 1876, James Thomson, brother of William Thomson, originated the concept of the differential analyzer, an analog computer designed to solve different equations using a wheel-and-disc system. More than 50 years later, the differential analyzer was built in 1927 by H.L. Hazen and Vannevar Bush. Many more analog computers were built over time and used for many purposes, such as simulations of aircraft, nuclear-power plants, and chemical processes. However, by the 1950’s, analog computers soon started becoming outdated due to the invention and success of early digital computers, ending the era of analog computing.
The Evolution of Computers 